In Ireland, unlike England or France, the majority of the fairy tales are, in fact, stories about fairies. When W.B. Yeats asked several contemporary Irish peasants if they believed in fairies, a typical answer was "Amn’t I annoyed with them". (Yeats, 3) The people researching their books on fairies had no difficulty finding Irish peasants who believed in fairies and followed certain superstitions in order to avoid their wrath, which was thought to be very nasty. The arrival of Christianity changed the way people thought about fairies but never destroyed the belief in them, or even tried to. Over the time the fairies changed from being nature gods to being a race of pesky little creatures, but in spite of these changes, there are many similarities between the Sidhe, or Tuatha De Danann, and the wee-folk of later legend.

There are many theories explaining the origins of the Irish fairies. The oldest is probably the one in W. Y. Evans Wentz’s book "The Fairy-Faith in Celtic Countries. He writes:

When the Sons of Mil, the ancestors of the Irish people, came to Ireland they found the Tuatha De Danann in full possession of the country. The Tuatha De Danann then retired before the invaders, without giving up their sacred Island. Assuming invisibility, with the power of at any time reappearing in a human-like form before the children of the Sons of Mil, the people of the Goddess Dana became and are the Fairy-Folk, the Sidhe of Irish mythology and romance. (Wentz, 284)

The Sons of Mil were the ancestors of the Celts. There is proof that there was a race of people living in Ireland before the Celts and for a very long time, perhaps even now, it was believed that these people shared Ireland with the Celts, living either underground, or invisibly. Some people are gifted with the second sight, the ability to see the Tuatha De Danann. They are generally described as being a race of tall, godlike people. They are sometimes seen as nature gods, responsible for the success of crops and dairy production. The men are generally described as being handsome warriors and the women are beautiful seductresses. Both men and women frequently seek humans to marry in the different myths and legends. They had a complex social structure, in which Dagda was high king and his sons held their own palaces. They were worshipped by the Celts, a form of ancestor worship.

Certain hills, fairy mounds, are held sacred and it is believed that one could be carried into the "Otherworld" if he/she fell asleep on one of these mounds. The Otherworld is also called "Tír-na-n-Og", which means "the Country of the Young." Wentz offers two different ideas as to where it might be located.

Sometimes, as is usual to-day in fairy-lore, it was a subterranean world entered through caverns, or hills, or mountains, and inhabited by many races and orders of invisible beings, such as demons, shades, fairies, or even gods…More frequently, in the old Irish manuscripts, the Celtic Otherworld was located in the midst of the Western Ocean, as though it were the ‘double’ of the lost Atlantis. (Wentz, 332-3)

The stories Yeats compiled reflect the later theory. He includes stories about people who see it appear in the ocean and try to sail to it but can not. According to Yeats, one man, Oisin the Bard, went to Tír-na-n-Og and lived to tell about it. Yeats writes:

The bard, Oisin, who wandered away on a white horse, moving on the surface of the foam with his fairy Niamh, lived there three hundred years, and then returned looking for his comrades. The moment his foot touched the earth, his three hundred years fell on him, and he was bowed double and his beard swept to the ground.

Yeats’ idea is consistent with that which states that the Sidhe are the main occupants of the Otherworld, and also the idea reflected in some of the stories Wentz includes. In these stories, those who return from the Otherworld must stay on horseback and never touch the earth if they want to return to Tír-na-n-Og.

The idea of the Celtic Otherworld is similar in many ways to the Greek and Roman Hades. Wentz compares the silver branch needed to enter the Otherworld with the golden bough needed by Aeneas to enter the underworld. There are different ideas of how the Celtic Otherworld relates to death. It is not believed that all people will go to the Otherworld when they die, as is the case with Hades. When infants, nursing mothers, and talented musicians die, it is believed that they are taken by the fairies to the Otherworld. King Arthur also rode a barge to the Otherworld, according to Arthurian legend. In classical mythology, there is a portion of Hades reserved for the truly extraordinary. The Otherworld is like that portion, but the qualities needed to enter it are measured by fairy standards. Wentz also writes about people who have lengthy illnesses spending that time with the fairies. When they regain their health, they are often gifted with the second sight. The belief in fairies existed simultaneously with both Druidism and Christianity. While the Otherworld is in many ways similar to Heaven, the ability of the living to enter it and its proximity to the mortal world set it apart. The idea of the Sidhe Otherworld has a greater parallel in Druidism, where the dead find rest in beautiful pleasant land, also called the Otherworld. There are both consistencies and differences between the Druid Otherworld and the home of the Sidhe, the main one being the relative unimportance of the Tuatha De Danann in the Druid Otherworld. Rather than it being their land, they remain on the outskirts, as they do in the land of the living. Yeats devotes a section of his book to the Otherworld.

The Christians thought of the Otherworld as being a kind of eternal purgatory, rather than a paradise, which is consistent with their conceptions of the fairies. While beliefs in the Sidhe remained, they were substantially altered as Ireland was converted to Christianity. The Christians felt no need to obliterate the Irish belief in the fairies, but they did insist that they no longer be worshipped as gods are. Wentz includes a quote from Standish O’Grady:

So firm was the hold which the ethnic gods of Ireland had taken upon the imagination and spiritual sensibilities of our ancestors that even the monks and christianized bards never thought of denying them. They doubtless forbade the people to worship them, but to root out the belief in their existence was so impossible that they could not even dispossess their minds of the conviction that the gods were real supernatural beings." (Wentz, 283)

While the Christians did not try to obliterate the belief in the People of the Goddess Dana, they did try to alter their nature to their own liking. Wentz tells of christianized stories in which the Tuatha De Danann are given demonic dispositions. The Christians also denied the immortality of the Sidhe so that they would be subject to the same death and judgment that humans are. Wentz writes "Not until Christianity gained its psychic triumph at Tara…did the Tuatha De Danann lose their immortal youthfulness in the eyes of the mortals and become subject to death." (Wentz, 292)

Over time, the very nature of the fairies began to change within a Christian framework. W. G. Wood-Martin illustrates how the fairies were, at least in the minds of the peasants, converted to a Christian way of thinking. He writes:

They (the fairies) are said to entertain, like many mortals, grave doubts regarding their future, although they have undefined hopes of being restored to happiness… Finvarra, the great fairy chief, once asked St. Columbkille if there were any hope that the fairies would gain heaven, but the saint answered that hope there was none, their doom was fixed, and at the great judgment day they would not merely die but would suffer annihilation. (Wood-Martin, 5-6)

In later times, other theories became more prevalent among the peasantry than that of the fairies being the Children of the Goddess Dana. Dana herself disappeared over time, first into the Celtic Goddess Brigid, and then into the Christian St. Bridget. As they stopped being worshipped, the fairies were no longer portrayed as being a race of gods. The integration of fairies into a Christian framework is explained in one of W. B. Yeats’ theories on the origins of fairies. In the introduction to his Fairy and Folk Tales of Ireland, W. B. Yeats asks of the fairies:

Who are they? "Fallen angels who were not good enough to be saved, nor bad enough to be lost", say the peasantry…Witness the nature of the creatures, their caprice, their way of being good to the good and evil to the evil, having every charm to them but conscience—consistency. (Yeats, 11)

This very Christian theory on the nature of the fairies is consistent with Wood-Martin’s discovery that, according to the common myths, fairies were preoccupied with salvation by the Christian God. If they had their origins in the Christian heaven, it is logical that they would aspire to regain their places there. Wood-Martin also suggests that the angels who neither sided with God, nor Lucifer in the war in Heaven were sent to earth, as being the middle ground, and then became the fairies. This theory fits the fairies neatly into Christian mythology. The Jewish tradition also integrates the fairies into their mythology. They say that the fairies are the children of Eve who she hid from God because they had not yet been washed. As a punishment, God made them invisible to the normal human eye.

The fairies are commonly known as the "good little people". There are called good to avoid incurring their wrath, which, as depicted in the fairy stories, is a terrible thing to do. It is compared to the way the Greeks refer to the Furies as the "Benevolent". The fairies are often depicted as being generous to those who do not come to them seeking to benefit and cruel to those who do. MacDougall and Calder tell a story about a hunchback who was tormented by the other children in his village. A fairy encounters him crying in the Willow Brake and she invites him into a fairy palace. When he returns home, a year and a day later, he is cured of his ailment. One of the boys who mocked him was also a hunchback. This boy, Punchy, forces the former hunchback to break his vow never to speak of the fairies. Punchy goes to the Willow Brake and asks to meet the fairy that helped the hunchback. The fairies become angry at Punchy for making the hunchback break his vow and also for seeking them uninvited. Instead of removing his hump, they double its size. This story is illustrative of the nature of fairies that Yeats describes. They show both their cruelty and generosity, giving both boys what they justly deserve. The peasants hoped that by referring to the fairies only as the "good people" or the "gentry" that they could avoid their punishments.

The fairies are also commonly referred to as the "wee folk". Wentz makes frequent references to the smallness of fairies. One of the peasants whom he spoke with said "One once appeared to me, and seemed only four feet high, and stoutly built. He said, ‘I am bigger than I appear to you now. We can make the old young, the big small, the small big.’" (Wentz, 47) This depiction is particularly appropriate for interpretations that say that fairies are made entirely of spirit, and therefore, all physical appearances are a façade. Some stories attribute fairies with glamour magic that they use to alter their appearance however they like. In most of the stories that deal with the fairies as being naughty little imps, they are depicted as being rather small and ugly. There is a recurrent story in which a man is terrified by a three-foot tall woman chasing him home. Fairies are also frequently called manikins, which implies that they are like men but smaller.

There are two distinct categories of fairy stories in Ireland, one group of which treats the fairies as the Tuatha De Danann, a race of gods, and another of which treats them as frequently bothersome imps. None of the stories are dated, but it can be assumed that they have all been passed down through generations and changed over time. Yeats describes a process by which the stories were regulated. Two storytellers would tell the same story, the audience would vote, and the preferred version was accepted as the prevailing one. It is unsafe to say that the stories of the Tuatha De Danann were unchanged by Christianity, but they bear more resemblance to Classical mythology, while the stories of changelings and imps bear more resemblance to Grimms Fairy Tales.

Wentz tells several stories of mortals who chose to live their lives in the fairy palaces with the Tuatha De Danann, rather than in the mortal world with the Sons of Mil. He tells the story of prince Laeghaire who went into a fairy mound with fifty of his warriors and decided to stay there forever. He and his warriors decide to return one final time to bid farewell to their families. They are told that they can only return to the fairy palace if they never dismount from their horses. There are also stories of men who fall in love with Sidhe women and return to the Otherworld to live with them. There are fewer, if any stories from Ireland of fairies who leave the Otherworld to live with mortal lovers.

The stories that treat the fairies as being pesky diminutive creatures deal largely with the abduction of babies and their replacements with little old fairies. Both the fairy sent in the place of the human child and the child sent to the Otherworld are referred to as changelings. Many of the stories tell of the ways people retrieved their own children from the fairies. Rather than being told from the point of view of the visitors to the Otherworld, these stories are told from the point of view of the mortals. One of the more common stories is that of the eggshell brewery. Yeats includes T. Crofton Croker’s version in his book. It starts with a woman noticing that her "healthy blue-eyed boy had become shriveled up into almost nothing, and never ceased squalling and crying." She was told to take some eggs, throw away the insides, and boil the shells in a pot. The result of her doing this was the child, who should have been too young to speak saying "I’m fifteen hundred years in the world, and I never saw a brewery of egg-shells before!" After this, the woman was free to chase the changeling out of the house with a red-hot poker, and her own child was then returned to her.

Part of the reason why the fairies’ demeanor was so changed, or at least varied, was the fact that the Christian priests discouraged people from worshipping the fairies and leaving them offerings. The most prevalent offering was dairy products, and while the practice of leaving some milk or butter for the fairies is still encouraged, the people who did not were subjected to various forms of punishment. Lady Gregory dedicates an entire chapter in her book to the subject of "Butter". She reports several examples of people who were unable to churn their butter due to fairy interference. Shakespeare’s Puck is guilty of such interference. A fairy asks him:

Either I mistake your shape and making quite,

Or else you are that shrewd and knavish sprite

Call'd Robin Goodfellow are not you he

That frights the maidens of the villagery;

Skim milk, and sometimes labour in the quern

And bootless make the breathless housewife churn;

(Midsummer Night’s Dream, II i)

To remedy against this, Lady Gregory suggests "To sprinkle a few drops of holy water about the churn, and to put a coal of fire under it, that you should always do—as was always done in the old time—and the others will never touch it." (Gregory, 247) She also tells of a woman who suggested getting some water from a running source. There is a superstition that says that the fairies can not cross running water.
Wentz explains the connection between the Tuatha De Danann and dairy. He writes:

Thus after their conquest these Sidhe or Tuatha De Danann in retaliation, and perhaps to show their power as agricultural gods, destroyed the wheat and milk of their conquerors, the Sons of Mil, as fairies today can do; and the Sons of Mil were constrained to make a treaty with their supreme king, Dagda, who…is himself called an earth God. Then when the treaty was made the Sons of Mil were once more able to gather the wheat in their fields and drink the milk of their cows; and we can suppose that their descendents, who are the people of Ireland, remembering that treaty, have continued to reverence the people of the Goddess Dana by pouring libations of milk to them and by making them offerings of the fruits of the Earth. (Wentz, 291)

Wood-Martin verifies the currency of this tradition by saying, "If a child accidentally spills her mug of milk on the ground, the mother says "That’s for the fairies: leave it to them and welcome." The child should not be reproved, for that would bring ill luck to the household." (Wood-Martin, 7)

While there are various explanations of the origins of fairies and the nature of them and their lands, there is no explanation of where the modern conception of fairies has come from. None of the books suggest that fairies have wings like dragonflies or butterflies. The wee-folk are generally thought to be the size of children or dwarfs, rather than the size of insects. They also tend to be suitably disproportionate, unlike the tiny but perfect adult fairies in modern storybooks. It is probable that these depictions of fairies sprang more from the minds of individual humans than any specific culture or mythology. Wentz mentions a wide variety of cultures that believe in fairies similar to either the Tuatha De Danann or the wee-folk, and of some cultures that see the fairies as the animistic spirits of nature. None of these fairies bear much resemblance to the modern fairies and if they had wings, it is a detail that Wentz leaves out. Spencer’s fairies were like the Tuatha De Danann, Shakespeare’s were like a combination of the Sidhe and the wee-folk, but it was not until the Victorian era that fairies were concretely established as little winged beings. One of the first of these fairies was probably Tinkerbell in J.M. Barrie’s Peter Pan. Around that time, there was also a large amount of sentimental art, creating cutesy portrayals of fairies and cherubs. There was also a large fuss made about the fairy photographs taken by two young girls in England.

In spite of the more recent depictions of fairies as beautiful fantastical creatures, there was nothing fictitious about fairies to the people of Ireland. Some of them believed that fairies were the Tuatha De Danann, former inhabitants of Ireland who became invisible and hidden from the eye of normal mortals. Others believed that the fairies were naughty little imps, responsible for crop failures and infant sicknesses. Even with the arrival of Christianity, the Irish people did not lose their faith in the fairies. They remained as real entities, subject to the same variance in moods and dispositions that could be found among humans. They had their own social structure and dwelling place, in a land as hidden from the mortal eye as its inhabitants are. While out of sight, the fairies remain an important part of Irish culture, inspiring stories and superstitions.