O'Rahilly's historical model

O'Rahilly's historical model is a theory of the history of early Ireland put forward by Celts scholar T. F. O'Rahilly in 1946. It was based on his study of the influences on the Irish language and a critical analysis of Irish mythology.

He distinguished four separate waves of Celtic invaders:

O'Rahilly's work was and remains influential but much of his linguistic work has since been refuted by noted authors such as Kenneth H. Jackson and John T. Koch and is not generally the accepted model.

According to O'Rahilly's model, between 700 and 500 BC, iron-wielding, Celtic-speaking people first settled in Great Britain and Ireland from the continent. They spoke a Brittonic language 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 the Picti "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, a Goidelic adaptation of the Brittonic word 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 Cruthin 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 County Laois, Leinster (formerly "Queen's County").

...
Wikipedia