The term "Cruthni" was applied to a number of early Irish population groups by the writers of Old and Middle Irish, although it is unclear whether it represents a self-description by these groups or whether it was an externally generated label. The word is an Irish cognate of Welsh Prydain and earlier Celtic Pretani, from which the modern English terms Britain and Britons derive. The equivalence of British "p" with Irish "kw," later "c," is well established, and the modern English forms in "b" derive from Latin forms themselves, based on a mishearing of the British pronunciation. Cruthni thus, in some sense, means "Britons," although by the Middle Ages the Irish distinguished between the descendants of the Romano-British population living south of the river Forth (in Scotland), whom they called Bretain (Britons), and the unromanized tribes to the north, whom they called Cruthni and who were known in Latin as Picti, the Picts. It is incorrect, however, to designate the Irish Cruthni as "Picts," for they were never so-called by Irish Latinists such as Adom-nan, who while using Picti for those in northern Britain, simply Latinized "Cruthni" when referring to the Irish.
Groups of Cruthni were found across Ireland, but only three groups were of any significance: the Sogain, a major subject people of the Connachta whose seven tribes were scattered across Connacht proper and Mide; the Lofgis (whose name survives as modern Laois) in western Leinster; and finally a large group of tuatha occupying most of Antrim and some of the neighboring districts in the northeast. It is with this last group that we shall principally be concerned. If the term "Cruthni" does imply an origin in Britain (the alternative view would be that the island takes its name from the people) then there is no reason to suppose any particular connection between the various Cruthnian groups within Ireland. Their ancestors may have made the crossing at different periods and from different places in Britain. Since the migrations are not recorded in the annals it is fairly certain that any such migrations happened before the fifth century a.d., perhaps long before.
While the Sogain, the Lofgis, and the smaller groups of Cruthni scattered throughout Ireland were for the most part loyal vassals within established over kingdoms, the northeasterners were a major force in their own right. Their territory in the mid-sixth century seems to have covered most of the land between Lough Foyle and the Lagan. They were bordered on the southeast by the Ulaid, on the southwest by the Airgialla, and on the west by the Northern Uf Neill. They were at this stage divided into many tuatha. At the battle of Moin Daire Lotheir in 563 two of the Cruthian tuatha under Baetan mac Cuinn, aided by the Uf Neill, fought against their over king Aed Brecc. Aed, together with seven of his allied kings, was slain, and at least one further Cruthnian, Eochaid Laeb, is said to have escaped the battle. This puts at least eleven kings of the Cruthni at this battle, giving some sense of their extent. Two years later Aed Brecc’s successor as Cruthnian over king, Aed Dub, slew Diarmait mac Cerbaill, king of Tara. In 637 the Cruthni were defeated by Domnall mac Aeda, king of Tara, at Mag Rath (Moira, County Down), and the battle became a central point of saga. These events of the decades around 600 give some indication of the importance of the Cruthni in the early period.
By the eighth century a single kingdom had emerged to dominate the Cruthni, that of the Dal nAraide of Mag Line (Antrim), and its name gradually drove that of the Cruthni from the record. In the later ninth and early tenth centuries Dal nAraide even began to exert their authority over the Ulaid, and they rewrote their own origin legends to make it appear that they had always been Ulaid.
Modern scholars have always stressed that there is no evidence of British speech or other cultural traits among the Irish Cruthni, but it must be conceded that we have no early texts emanating from their province and that the record of their personal names, and so forth, may have been normalized by the chroniclers, just as they were able to produce Gaelic forms of the names of Pictish kings in Britain whose own preferred spelling forms are preserved elsewhere. This said, had any migration occurred before the fifth century it is quite likely that British and Irish Celtic would have been close enough that convergence of dialects rather than language shift would have been required to bring the two tongues together.
A further problem regards the relationship between the Cruthni and Dal Riata. By the seventh century the kingdom of Dal Riata had emerged, extending from the valley of the Bush in Antrim as far as Mull and the adjacent mainland in Scotland, with ecclesiastical centers at Armoy, Kingarth, and Iona and royal centers at Dunseverick, Dunadd, and Dunollie. In Ireland, Dal Riata was completely enclosed by Cruthnian tuatha, while in Scotland it appears to have been the beachhead from which Gaelic language and culture was eventually to spread to the whole region. At the end of the sixth century Aedan mac Gabrain, king of Dal Riata, seems to have displaced Aed Dub as over king of the Cruthni and become a dominant figure in the north of both Britain and Ireland. Later genealogists gave Dal Riata a distinct descent from the Cruthni, making them either exiles from the Dingle peninsula or a branch of the Ulaid, but the close geographical and political relationship between the two groups begs some questions. It is also odd that the Irish population group described as "Britons" should be such close neighbors of a population group in Britain perceived as Irish by their neighbors. Possibly both groups represent the two halves of a people who bridged the gap between Britain and Ireland, in cultural as well as geographical terms.
In religious terms, the most important Cruthnian saint was Comgall, founder of Bangor and friend of Colum Cille. His foundation at Bangor lay on the borderlands between the Ulaid and the Cruthni.
