Tirkeeran derives its name from the territory of the Airgiallan clan; Ui Mhic Carthainn, one of the earliest tribes in the area based to the south-east of Locha Febail. The Ui Mhic Carthainn are claimed to descend from Forgo mac Carthainn, the great-great-grandson of Colla Uais. Dunchad mac Ultain is cited as king of the Ui Meic Cairthinn in 677, when he was killed by the Cenél nEóghain king, Mael Fithrich. It was around this period that the Cenél nEóghain were attempting to expand eastwards from their homeland in Innishowen, in modern-day County Donegal and into central Ulster. Eventually the Cenél nEóghain crushed the power and independence of the Ui Mhic Carthainn and subjugated them, however it would remain in some minor form until the end of the eleventh century when the Annals cite Conchobhor ua hAinniaraidh, king of Cianachta and Ua Cein king of the Ui Meic Carithinn fell by each other in combat. By the 12th century, the area of Tirkeeran was divided into three districts, given as belonging to the Ó Eiteagain in the 12th century: Teallach Caghalain; Teallach Duibhailbe; and Teallach Braeain. Prior to the 13th century the septs of Ó Cairealláin and Mac Eiteagain are cited as ruling the territory of Clann Diarmatta, which contained the present-day parish of Clondermot. These septs descend from the Cenél nEóghain branch Clann Conchúir Magh Ithe, of which the O'Cahans were the principal sept and who would later dominate and rule a territory spanning the baronies of Tirkeeran, Keenaght, and Coleraine. When the baronies of Ulster were being created by the English from 1585, the general manner was to name it after the principal town or castle lying within the area, in which they held their court, baron, and gaol. This resulted in Firnacreeve being renamed as the barony of Coleraine, and Kinel-Ferady to the barony of Clogher. The 1591 inquisition which shired County Tyrone specified as one of its eight baronies "Anagh, conteyninge Tyrchyrine , Sgryn , and Clandermod ". The name Anagh was after the O'Cahan's castle, which stood on an island in Enagh Lough. Anagh was one of three baronies transferred from Tyrone to the new County Coleraine, which in turn was enlarged and renamed County Londonderry in 1613. The name of the barony was changed from Anagh to Tirkeeran some time between 1615 and 1639.
