Celtic is the mythology of Celtic polytheism, the religion of the Iron Age Celts. Like other Iron Age Europeans, the early Celts maintained a polytheistic mythology and religious structure. For Celts in close contact with Ancient Rome, such as the Gauls and Celtiberians, their mythology did not survive the Roman Empire, their subsequent conversion to Christianity and the loss of their Celtic languages. It is mostly through contemporary Roman and Christian sources that their mythology has been preserved.
The Celtic peoples who maintained either political or linguistic identities (such as the Gaels in Ireland and Scotland, the Welsh in Wales, and the Celtic Britons of southern Great Britain and Brittany) left vestigial remnants of their ancestral mythologies that were put into written from during the Middle Ages. Although the Celtic world at its height covered much of western and central Europe, it was not politically unified nor was there any substantial central source of cultural influence or homogeneity; as a result, there was a great deal of variation in local practices of Celtic religion (although certain motifs, for example the god Lugh, appear to have diffused throughout the Celtic world).
Inscriptions of more than three hundred deities, often equated with their Roman counterparts, have survived, but of these most appear to have been genii locorum, local or tribal gods, and few were widely worshiped. However, from what has survived of Celtic mythology, it is possible to discern commonalities which hint at a more unified pantheon than is often given credit. The nature and functions of these ancient gods can be deduced from their names, the location of their inscriptions, their iconography, the Roman gods they are equated with, and similar figures from later bodies of Celtic mythology.
Celtic mythology is found in a number of distinct, if related, subgroups, largely corresponding to the branches of the Celtic languages:
- Ancient Celtic religion (known primarily through archaeological sources rather than through written mythology).
- Mythology in Goidelic languages, represented chiefly by Irish mythology (also shared with Scottish mythology.
- Mythological Cycle
- Ulster Cycle
- Fenian Cycle
- Cycles of the Kings
- Mythology in Brittonic languages:
- Welsh mythology
- Cornish mythology
- Breton mythology
