From Wikipedia

Legends of the Celts

The earliest directly attested examples of a Celtic language are the Lepontic inscriptions, beginning from the 6th century BC. Continental Celtic languages are attested only in inscriptions and place-names. Insular Celtic is attested from about the 4th century AD in ogham inscriptions, although it is clearly much earlier. Literary tradition begins with Old Irish from about the 8th century. Coherent texts of Early Irish literature, such as the Táin Bó Cúailnge (The Cattle Raid of Cooley), survive in 12th-century recensions. According to the theory of John T. Koch and others, the Tartessian language may have been the earliest directly attested Celtic language with the Tartessian written script used in the inscriptions based on a version of a Phoenician script in use around 825 BC.