The Pretanic Colonisation
Between 700 and 500 BC, iron-wielding, Celtic-speaking people first settled in Britain and Ireland from the continent. They spoke a P-Celtic tongue, and called themselves Priteni or Pritani. The impact they had upon the native inhabitants can be inferred from the fact that Greek geographers were referring to these islands as the “Pretanic Islands” (αι Πρετανικαι νησοι) by at least 300 BC. It is also possible that the very name “Britain” is derived from Priteni. However, there is no hard evidence for a Pretanic invasion as such. It is much more likely that their settlement of these islands was a gradual one, spread over several centuries.
In Britain these Priteni were absorbed by later invaders and lost their cultural identity, except in the far north where they were known to the Romans as Picti, or “painted people,” on account of their practice of decorating their bodies with tattoos (a practice which by then had died out among other Celtic nations). In Ireland, too, the Priteni were largely absorbed by later settlers; but a few pockets of them managed to retain a measure of cultural, if not political, independence well into the Christian era. By then they were identified as Cruithne or Cruthin, a Q-Celtic adaptation of the P-Celtic Priteni. Both words are derived from a root meaning “to shape” or “create.” Celtic tribes generally gave themselves names which were the pluralised forms of names they gave to their deities (in this case “the Creator”). Among the Cruthnian tribes that survived into the Christian era the most prominent were the Dál nAraidi in Ulster, and the Loíges and Fothairt in Leinster. The name of the second of these tribes, modernized as Laois, has been revived and given to one of the counties of Leinster (formerly known as Queen's County).
Read more about this topic: O'Rahilly's Historical Model