What is "airgialla"

Airgíalla ( Modern Irish: Oirialla, English: Oriel) is the name of a medieval Irish over-kingdom and the collective name for the confederation of tribes that formed it. The confederation consisted of nine minor-kingdoms, all independent of each other but paying nominal suzerainty to an overking, usually from the most powerful dynasty. Airgíalla at its peak spanned the modern Irish counties of Londonderry, Monaghan, Tyrone, along with parts of Armagh, Fermanagh and Louth. The later constricted kingdom of Airgíalla survived in Monaghan—which was known as Oirghialla and Oriel after the Norman Invasion of Ireland—until the end of the Gaelic order in Ireland.

Airgíalla was according to legend founded by the Three Collas, who conquered what is now central Ulster from the Ulaid. The decisive victory was the battle of Achadh Leithdheirg, said to have been fought around the year 331. The actual year and circumstances of how the Airgíalla confederation came into existence is however unknown.

Originally thought to have been under the dominance of the neighbouring Ulaid to the east, the territory of the Airgíalla from the 6th-century onwards was gradually eroded by the encroachment of their western neighbours, the Cenél nEógain of the Northern Uí Néill, as well as the Southern Uí Néill to their south. From 735 they fell under the dominance of the Cenél nEógain, and by 827 had become their vassals.