From Wikipedia

The Primitive Celts

Celts  is a modern term used to describe any of the European peoples of antiquity who spoke a Celtic language.

The historical Celts were a diverse group of tribal societies in Iron Age Europe. Proto-Celtic culture formed in the Early Iron Age (1200 BC-400 AD) in Central Europe (Hallstatt period, named for the site in present-day Austria). By the later Iron Age (La Tène period), Celts had expanded over a wide range of lands: to the British Isles (Insular Celts), the Iberian Peninsula (Celtiberians), much of Central Europe (Gauls) and following the Gallic invasion of the Balkans in 279 BC as far east as central Anatolia (Galatians).

The earliest direct attestation 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. 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.

By the early first millennium AD, following the expansion of the Roman Empire and the Great Migrations (Migration Period) of Germanic peoples, Celtic culture had become restricted to that of British Isles (Insular Celtic), and the Continental Celtic languages ceased to be widely used by the sixth century.

Insular Celtic culture diversified into that of the Gaels, the Welsh and the Bretons of the medieval and modern periods. A modern "Celtic identity" was constructed in the context of the Romanticist Celtic Revival, mostly in Great Britain and Ireland.

Diachronic distribution of Celtic peoples:
     core Hallstatt territory, by the sixth century BC      maximal Celtic expansion, by the third century BC      Lusitanian area of Iberia where Celtic presence is uncertain      the "six Celtic nations" which retained significant numbers of Celtic speakers into the Early Modern period      areas where Celtic languages remain widely spoken today