Airgialla, "those who give hostages," was a collective name for a group of peoples around the Sperrin Mountains in the north of Ireland and in the midlands. They consisted of nine main tribal groups: Ui Maic Cafrthinn, south of Lough Foyle; Ui Fiachrach of Ardstraw; Ui Thuirtri east of the Sperrins (collectively known as Ui Maccu Uais); the Fir Chraibe and the Fir Li west of the Bann; the Airthir around Armagh; the Ui Chremthainn in Fermanagh, parts of Tyrone and Monaghan; the Ui Meith in Monaghan; and the Mugdorna, who also stretched into Meath. Branches of the Ui Moccu Uais were in Westmeath and Meath also. It is possible that the Deisi around Tara were Airgialla. Originally probably subject to the Ulaid (Ulstermen) they were gradually, from the sixth century onwards, brought under the control of the Ui Neill, especially by the Cenel nEogain who were expanding from their homeland in Inishowen across Lough Foyle and eastwards across Counties Derry and Tyrone.
Following the defeat of the Ulaid in the battle of Mag Roth (Moira, County Down) in 637 to 638, they enjoyed a degree of independence from both their former masters and the expanding Cenel nEogain. After the devastating defeat of the Ulaid at the battle of Fochairt in 735, the Cenel nEogain dominated the Airgiallan territories from the shores of Lough Foyle to the coast of Louth. The Airgialla provided military service for the Ui Neill and propaganda was produced explaining their evolving relationship with them. Defeated in the battle of Leth Cam beside Armagh in 827, they became vassals of the Cenel nEogain. It is very likely that it was Airgiallan patronage that helped Armagh rise to power during the seventh century to become the chief church in Ireland. The Airthir ("Easterners") had control of the offices in the church of Armagh. The Clann Sfnaigh monopolized the abbacy from 996 until the twelfth century. The Ui Thuirtri migrated east of the Bann from 776 onwards and lost their link with the Airgialla after 919. The Airgiallan peoples in the midlands were absorbed by the various branches of the Southern Ui Neill. As the northern and southern branches of the Ui Neill drifted apart, two kingdoms emerged as a wedge between them. In Counties Leitrim and Cavan, the kingdom of Ui Briuin Breifne (later O’Rourkes) was formed. Parallel with this kingdom to the north in Counties Fermanagh and Monaghan and parts of Louth, a consolidated kingdom of Airgialla emerged, and partly as a result of continuing pressure from the Cenel nEogain, who absorbed their northern borders, they moved toward the southeast. By the eleventh century the leading family was Ua Cerbaill (O’Carroll). Donnchad Ua Cerbaill pushed the southern boundaries of this kingdom to the Boyne in the twelfth century and had the seat of the diocese of Clogher transferred to his power center in Louth. When the Anglo-Normans conquered Louth, this area became known as "English Oriel" and this portion of O’Car-roll’s kingdom was transferred to the diocese of Armagh. The diocese of Clogher represents the medieval kingdom of Airgialla.