The Cruthin (ˈkɾɣʊθjɪn̠j; Cruithnig or Cruithni; Cruithne ˈkɾɣɪ(h)njə) were a people of early medieval Ireland. Their heartland was in Ulster and included parts of the present-day counties of Antrim, Down and Londonderry. They are also said to have lived in parts of Leinster and Connacht. Their name is the Irish equivalent of *Pritanī, the reconstructed native name of the Celtic Britons, and Cruthin was sometimes used to refer to the Picts, but there is a debate among scholars as to the relationship of the Cruthin with the Britons and Picts.
The Cruthin comprised several túatha (territories), which included the Dál nAraidi of County Antrim and the Uí Echach Cobo of County Down. Early sources distinguish between the Cruthin and the Ulaid, who gave their name to the over-kingdom, although the Dál nAraidi would later claim in their genealogies to be na fír Ulaid, "the true Ulaid". The Loígis, who gave their name to County Laois in Leinster, and the Sogain of Leinster and Connacht, are also claimed as Cruthin in early Irish genealogies.
By 773 AD, the annals had stopped using the term Cruthin in favour of the term Dál nAraidi, who had secured their over-kingship of the Cruthin.
Britain (placename)
In medieval Irish writings, the plural form of the name is variously spelt Cruthin, Cruithin, Cruthini, Cruthni, Cruithni or Cruithini (modern Irish: Cruithne). The singular form is Cruithen (modern Irish: Cruithean). The adjectival form is Cruithnech (modern Irish: Cruithneach), which is also used as a noun. It is thought to relate to the Irish word cruth, meaning "form, figure, shape". The name is believed to derive from *Qritani, a reconstructed Goidelic/Q-Celtic version of the Brittonic/P-Celtic *Pritani. Ancient Greek geographer Pytheas called the Celtic Britons the Pretanoí, which became Britanni in Latin.
It is suggested that Cruthin was not what the people called themselves, but was what their neighbours called them.