

Insular Celtic culture diversified into that of the Gaels (Irish, Scots and Manx) and the Celtic Britons (Welsh, Cornish, and Bretons) of the medieval and modern periods. They followed an ancient Celtic religion overseen by druids. Most written evidence of the early Celts comes from Greco-Roman writers, who often grouped the Celts as barbarian tribes. Elements of Celtic mythology are recorded in early Irish and early Welsh literature.

Celtic literary tradition begins with Old Irish texts around the 8th century AD. Insular Celtic languages are attested from the 4th century AD in Ogham inscriptions, although they were clearly being spoken much earlier. Continental Celtic languages are attested almost exclusively through inscriptions and place-names. The earliest undisputed examples of Celtic language are the Lepontic inscriptions from the 6th century BC. In particular, there is dispute over the ways in which the Iron Age inhabitants of Britain and Ireland should be regarded as Celts. The relationship between ethnicity, language and culture in the Celtic world is unclear and controversial. Historic Celtic groups included the Gauls, Celtiberians, Gallaecians, Galatians, Britons, Gaels, and their offshoots.
