Among the pre-Milesian tribes of Connacht
the Gregraige, a Firbolg tribe, that inhabited much of the western part of present day County Sligo between Loch Gara and the Ox mountains. Located in western Co. Cavan and northern Co. Meath, the Mugdorna occupied this territory of the Fir Roiss prior to 800 AD, when they were pushed out of Meath and into Monaghan and replaced by the Gailenga.The Gailenga Mora left their name in the barony of Morgallion (Machaire Gailenga, "the plain of the Gailenga") in County Meath.
The Clann Cholgan included the families of MacColgan, O’Hennessy and O’Holohan. The MacColgans (Mac Colgan) were chiefs of the territory around Kilcolgan in the extreme northeast of County Offaly. A branch of the O’Hennessys were chiefs of Gailenga Beg, the district between Dublin and Tara, until they were dispersed into Offaly as a result of the Anglo-Norman invasion.
The Uí Maic Uais, descended from one of the Three Collas; the Delbna septs; the Gailenga Brega, the Luigni Mide and Fir Chul, the Saithne Brega, and the Mugdorma, among others.
Other tribes sharing the same area of County Sligo were the Gailenga and the Luigne (Connacht area), tribes of the Tuatha de Danann Celts from which O'Hara and O'Gara are descended; Dalriada.
The Ciarrage tribes or "black people" populated much of northwestern County Roscommon and are believed to have been the early Lords of Airtech, an area corresponding to the present-day barony of Frenchpark. Their seate was believed to be at Baslic near Castlerea.
The Calraige, another important tribe, had lands in Sligo and Mayo and north Roscommon. They may have been the rulers of Moylurg who were in later centuries absorbed by the expanding Síl Murray (later the MacDermots).
The Cianacht encompassed the O’Connors (O Conchobhair) of Keenaght, and the Luighne. The O’Connors were lords of Keenaght, County Derry, until dispossessed by the O’Kanes shortly before the Anglo-Norman invasion in the twelfth century. The Rudrician Families from the reign of 'Roderick the Great' or 'Rory the Great'.