The Gaelic (Q-Celtic hypothesis) name Cruithne is cognate with the P-Celtic Pruteni (recorded in Ancient Greek as pryteni) (also Pritani), from which derives the name Pretannike (Latin Britannia), used as the term for Britain by Pytheas in about 325 BC. Today they are referred to as the Ancient Britons, whose nearest cultural descendants are the Welsh. The medieval Vulgar Latin form Bretani is the origin in the modern Irish form "Breathnach", which also means Welsh.
However T. F. O'Rahilly in his historical model supposes that these Priteni were the first Celtic group to inhabit Britain and Ireland, and identifies them with the Picts of Scotland. They settled in Britain and Ireland between 700 and 500 BC. Around 50 BC Diodorus wrote of "those of the Pretani who inhabit the country called Iris (Ireland)". Whether the Priteni can be considered Celts in the linguistic sense thus depends on the classification of the Pictish language.
The first reference to the name Pict is found in a Latin document dated AD 297.
Among the Cruthnian tribes that survived in Ireland were the Loíges and Fothairt in Leinster. The name of the first of these tribes survives in the modern form of Laois as the name of one of the counties of Leinster. The other main group lived in County Down, became allied to the Dal Fiatach kingdom, and are ancestral to the chiefs of the Magennis and McCartan clans. The Annals of Ulster refers to a deceased County Down man in 698 as "nepos Predani", the Latin form of a "grandson of the Cruithne man". Despite a separate ethnic sense, the Cruithne / Cruthin groups were culturally a part of the Gaelic world at the arrival of Christianity and writing c.500AD.