According to legend, Airgíalla was founded by the Three Collas, who are said to have 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. However, this tale is thought to be mostly fiction, and the actual year and circumstances of how the Airgíalla confederation came about is 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 northern 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. The kingdom of Airgíalla was at its peak in the 12th century, under king Donnchad Ua Cerbaill. The later constricted kingdom of Airgíalla survived in Monaghan—which was known as Oirghialla and Oriel after the Norman Invasion of Ireland—under the Mac Mathghamhna, until the end of the Gaelic order in Ireland.
Airgíalla may mean "those who give hostages" or "the hostage givers", and refers to both the Irish over-kingdom of Airgíalla, and the confederation of tribes that formed it. It is commonly Anglicised as Oriel; however, archaic Angliciations include: Uriel, Orial, Orgialla, Orgiall, Oryallia, and Ergallia.
After the Anglo-Norman invasion, the Anglicisation "Uriel" became the name of the part of Airgíalla that had extended into modern-day County Louth. Similarly, the portion of Airgíalla that survived in modern-day County Monaghan, became known as Oirghialla, from which derives the Anglicisation "Oriel".
In early manuscripts the Bishop of Clogher was styled Bishop of Oirialla.