From The Art and Popular Culture Encyclopedia
Celtic mythology is the mythology of Celtic polytheism, apparently the religion of the Iron Age Celts. Like other Iron Age Europeans, the early Celts maintained a polytheistic mythology and religious structure. Among Celts peoples 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 Celts peoples who maintained either their political or linguistic identities (such as the Gaels and Brythonic tribes of Great Britain and Ireland) left vestigial remnants of their forebears' mythologies, put into written form during the Middle Ages.