Below is a brief mythological history of Ireland, a somewhat similar history may be found in the Welsh literature for the island of Britain although this is less clearly defined than that of the neighbouring island.
"The Lebor Gabála, the 'Book of Conquests', tells of successive invaders of Ireland, an account slightly modified by suitable obeisances to orthodox Christianity but retaining much of the flavour of pre-Christian times.
"The first race which inhabited Ireland perished in the biblical Flood. It was followed 268 years later (PK - 2048 BC?), on the first of May, by a group of 24 males and 24 females led by Partholón. At that time there were in Ireland only one treeless and grassless plain, three lakes and nine rivers, but during Partholón's time four new plains were cleared and seven new lakes were formed. Before his time there had been no tilling of the soil. After three centuries the population had grown to 5,000 but on the tercentenary of Partholón's landing his people were wiped out by an epidemic, gathering together to die on the original plain in Ireland. Although there were no survivors the knowledge brought and augmented by Partholón's people did not perish, the knowledge and working of gold, the first brewing of beer, the first cauldron and the introduction of domesticated cattle. To this period are also attributed some of the less tangible assets of civilisation, law-giving and ritual practices. As did their successors, so did the people of Partholón fight against and defeat the Formorians. These latter were a race of demons, generally monstrous and hideous, who fought against Partholón with supernatural powers.
"After an interval of 30 years the people of Nemed came into Ireland (PK - 1718 BC?) and in their time the face of the countryside was again changed by the clearance of twelve new plains and the formation of four new lakes. Decimated by the same epidemic which had annihilated Partholón they were unable to defend themselves adequately against the Formorians and became their vassals. Part of their tribute was the delivery on the first of November of two-thirds of the children born to them each year, two-thirds of their corn and their milk. After a battle with the Formorians in which Conann and many Formorian followers were killed, the remnants of the people of Nemed fled from the country.
"Next followed on the first of August the Fir Bolg together with the Fir Gaileoin and Fir Domnann. The similarity of these names to those of the Belgae, Gauls and the Dumnonii, has suggested that this 'invasion' refers to the arrival of certain tribes of the proto-historic period. Whether or not these are to be in any way connected with the Celtic people is uncertain, but their mythical contribution to the cumulative wealth of the country lies in the warlike sphere of their armament and the aristocratic notion of monarchy. Their rule did not remain undisputed for long although they were not attacked by the Formorians, but they were soon dispossessed by the Tuatha Dé Danann, the People of the Goddess Danu.
"The Tuatha Dé Danann landed on the first of May and after some unsuccessful negotiations with the Fir Bolg battle was joined at Mag Tuireadh (PK - the Tuatha Dé Danann or 'the folk of the God whose mother is Danu' were a confederacy of tribes in which the kingship went by matrilinear succession, some of whom invaded Ireland from Britain in the middle Bronze Age. Originally forced from Greece by invasions from Syria, reaching Ireland from Denmark, to which they gave their own name 'The kingdom of the Dananns' and north Britain which one date cites their arrival at in 1472 BC). The Tuatha were victorious and allowed the conquered to retain the Province of Connacht while they took possession of the remainder of the island, building their capital at Tara. Still unconquered the Formorians disputed the ownership of the land of Ireland but the Tuatha, perhaps recognising the strength of their ancient powers, attempted an alliance. During the battle of Mag Tuireadh Nuada, king of the Tuatha, had lost his right hand and, as a king had to be without physical blemish, he was obliged to abdicate. (PK - interesting then that today we have Hugh O'Neils Red Hand of Ulster). In his place Bres, the son of a Formorian father and a mother from the Tuatha Dé Danann, was elected and the alliance further strengthened by dynastic marriages, including that of Bres to Brigit, the daughter of the Dagda, one of the chieftains of the Tuatha. Despite these precautions the alliance was uneasy, aggravated by the lack in Bres of the generosity demanded of a king of the Tuatha and his imposition of excessive taxes. Eventually Bres in his turn lost his eligibility for the kingship having been so satirised so successfully by Cairbre, the principal bard of the Tuatha, that boils appeared on his face. His enforced abdication resulted in formal war between the Tuatha and the Formorians, a war fought after seven years of preparation with the help of magical weapons. The Formorians were defeated at the second battle of Mag Tuireadh, or Moytura the Northern, used to distinguish it from the earlier battle of the same name.
"The Tuatha themselves, however, were destined in turn to be dispossessed by the last race to take possession of Ireland, the Sons of Mil, the Milesians. The latter's arrival (PK - ~1000 BC?) on May the first and the subsequent battle for supremacy of the island was attended by formal and ritual observances, similar to those noticed in the conflict between the Tuatha and the Formorians. Similarly, magical powers were used by both sides but in two successive battles the Tuatha were defeated and, according to popular tradition, made terms with their conquerors. The Lebor Gabála states that they were expelled from the island but this is in contradiction to the remainder of Irish tradition. The Tuatha became the gods of the Celts and the majority retired to the síde, the prehistoric burial mounds of the country."
New Larousse Encyclopedia of Mythology, 1973, pp224-226
Another interesting source is Robert Graves' 'The White Goddess'. Later incursions included those of the Vikings and the English.
Many ancient sites appear to be designed to accommodate large crowds who, obviously, are expected at the same time of year. With the renewal of the solar year, the mid-winter solstice, the significance has been carried down even to today, Mithras having being superceded by the Roman Christ.