Among the most important concepts of Celtic spirituality is that of the sovereignity of Nature. Everything in nature is connected to its own spirit, also connected to the gods and thru them the people. In the past the kings were married to the land thru ceremony, joining him with the Goddess and the land belonging to her. Springs and wells were regarded as connected to the Underworld, where the ancestors and the gods (the sidhe) dwelled. They were gateways to the the spiritual otherworld. Worship took place amongst the trees, in sacred groves called "nemetons". Even the very cosmos itself was seen as forces of nature; the Land, the Sea and the Sky. The "Bilios" or World Tree represented the cosmos; the roots reaching into the Underground, the trunk of the mortal world, and the branches reaching into the celestial.
Within this concept of the cosmos we begin to see the importance of triplicity in Celtic thought. Some of the god/goddesses had three aspects, either in their attributes or their personifications. The druids were divided into three orders. Their logic itself was often taught by threes, in the form of poetic triads.
The cosmos is considered to be in balance, all aspects of spirit, nature, gods and humanity co-existing. The world of spirit, or the otherworld, is interwoven with nature and connected to the physical universe. It can be accessed thru communing with nature or the nature spirits, at sites or times seen as thresholds between the physical and spirit worlds. Examples of which would be : dusk, dawn, caves, springs and wells, barrows and standing stones. The mind itself also reaches into the otherworld thru meditation and ritual.
There are three realms spoken of as the whole of the cosmos: the Sky, from which the Tuatha De Danann descended, is the realm of the gods. It is a realm of order, from which culture and enlightenment descends. The Sea, which is below us merging with the Underworld, encircles the Earth and supports it. It is the land where the ancestors dwell and is also where the soul abides before birth and after death. The Land, where mortals dwell, is the meeting place of Sky and Sea, above and below. It is believed that these Realms overlap, and traverse between them is possible.
The directions of north, south, east and west are not spiritual realms, but represent the four lands from which the four treasures of the gods were given, and the mortal functions associated with them. From the west and the land of Goirias comes the Spear of Lugh and with it, knowledge. From the north, the land of Finnias comes the Sword of Nuada and battle, or justice and protection. From the east, the land of Muirias, comes the Cauldron of the Dagda (the father of many of the gods) and the gift of plenty. And from the south and the land of Failias comes the Stone of Fal (the stone that cries out upon the touch of the rightful king) and the gift of artistic inspiration.
The pagan Celts were polytheistic. One of the reasons stated for the overwhelming number of Celtic gods is that each tribe had its own dieties and local land spirits. There were gods that were commmon to the entire culture, but they were relatively few. Altho polytheistic, the Celts did have a fondness for depicting their dieties as couples, which is confusing when viewed alongside the duotheistic concepts of modern Wicca, the most familiar paradigm to modern pagans. What needs to be remembered is that each god and goddess were individuals unto themselves, and not an embodiment of a single Lord and Lady.
The Celts celebrated the seasonal festivals. Samhain, the last harvest and winter's beginning; Imbolc at springs return; Beltaine, the return of the sun and the onset of summer; and Lughnasa, the first fall harvest. Altho there are no records of the Celts celebrating the soltices and equinoxes they almost certainly would have been aware of these date's significance. It is not a stretch to incorporate these dates into completing a Wheel of the Year, with celebrations thruout.
Much more could be written, but it is not the intention to give a full exposition of Celtic spirituality. Only a few ideas and concepts have been presented to give the reader an overview of some themes. If the reader wishes to study further, there are many links presented on the final page that will lead you to a deeper understanding of Celtic tradition, in both past and present.