Celtic mythology is known to us only in the fragments that have come down to us through Irish (Gaelic) and Welsh (Brythonic) romances. Of the mythology of the Continental Celts we know nothing:
On the Continent the Celtic tribes came in contact with the rich and highly organised Graeco-Roman mythology, and discarded their own mythic romance. In the British Isles Celtic mythic romance escaped the destructive influence of Rome, was spared by Christianity, and served, almost down to the present day, as a backbone and rallying centre to the peasant lore about the fairies, which is substantially the old agricultural faith, preserved in rude and crude form, and partly reshaped by the fierce opposition or the insidious patronage of Christianity. Gaelic peasant lore only differs from that of other parts of Europe, because Gaeldom has preserved, in a romantic form, a portion of the pre-Christian mythology. Thanks to the fact that this mythology enters largely into the Arthurian romance, the literature of modern England has retained access to the fairy realm, and has been enabled to pluck in the old wonder-garden of unending joy fruits of imperishable beauty. 15
In Ireland a learned class who took pride in preserving the relics of the national past, wrote down histories and romances that contained mythological material. We have these histories and romances in documents of the eleventh and twelfth centuries, the Book of Leinster and the Book of the Dun Cow: the material on which they are based is of a much earlier period. In Wales a material less copious and more distorted, was, between 1080 and 1260, shaped into the romances that we know in the Mabinogion.
The Celts were known in the ancient world for their positive beliefs concerning the survival of the soul. They appear to have had a conception of a Happy Otherworld which was similar to that of the early Greeks:
Although from fifteen hundred to two thousand years separate the earliest recorded Greek and Irish utterances in a form, substantially speaking, yet extant, yet both stand on much the same stage of development, save that Ireland has preserved, with greater fulness and precision, a conception out of which Homeric Greece had already emerged. Examination of the mythologies due to other Aryan races, or rather, to prejudge nothing, to peoples speaking Aryan tongues equally with the Greeks and Irish, reveals the remarkable fact that Greeks and Irish alone have preserved the early stage of the Happy Other. world conception in any fulness. 16
The Celtic religion appears to have been the worship of the Powers of Life and Increase:
In Greece the Powers of Life and Increase, worshipped by the primitive agriculturists, are but one element in the completed Hellenic Pantheon, and this has been subjected to so much change, to such enlargement and glorification, as to be well-nigh unrecognizable. In Ireland, to judge by extant native texts, these powers must have constituted the predominant element of the Pantheon, and cannot have departed very widely from their primitive form. . . . In the main that mythology had for its dramatis personae the agricultural Powers of Life and Increase, in the main it was made up of stories of which the ultimate essence and significance were agricultural. 17
The same authority offers the following conclusions on the subject of Celtic mythology as it is revealed in the Irish romances:
The features common to Greek and Irish mythology belong to the earlier known stage of Aryan mythical evolution, and are not the result of influence exercised by the more upon the less advanced race. Survivals in Greece, they represent the high-water mark of Irish pre-Christian development; hence their greater consistency and vividness in Ireland. Fragmentary as they may be in form and distorted as it may be by its transmission through Christian hands, we thus owe to Ireland the preservation of mythical conceptions and visions more archaic in substance if far later in record than the great mythologies of Greece and Vedic India. 18
The Celtic stories given here deal mainly with adventures in the Happy Otherworld, in the Divine Land. The Voyage of Prince Bran is a typical story. Translated by Kuno Meyer, it is published with a comment by Alfred Nutt which is a study of Celtic mythology. The poems form the oldest part of the story; they date back to the eighth, or possibly to the seventh century. In the Divine Land to which Prince Bran voyages take place the events which lead up to the birth of Etain and afterwards to the death of King Conaire. This Divine Land is also the scene of Pwyll's adventures in the Welsh story. Pwyll, Arawn, and Mathonwy were originally divinities in Celtic Britain: their stories are taken from the Welsh Mabinogion.
xvii:15 Alfred Nutt: The Voyage of Bran, Vol. 2, in the Grimm Library.
xviii:17 Alfred Nutt: The Voyage of Bran, Vol. 2, in the Grimm Library.