The Airghialla were a loose federation of tuath, largely located in the modern province of Ulster.
By the 4th and 5th centuries much of the western and central portions of Ulster began to be dominated by the tribes of the northern Uí Neill and the Oirghialla. Their territory, depending on timeframe, included much of what includes the modern counties of Monaghan, Armagh and Fermanagh. In addition, other places where Arghialla groups have been noted included areas within the modern counties of Tyrone, Cavan, Meath, Westmeath, Louth, and Derry. The Oirghialla helped the Cenél nEoghain of the Uí Neill effect the conquest of most of North Ireland from the Ulster Érainn, of Argyll and Dalriada and later they settled a vast territory there including the counties of Louth, Armagh, Monaghan and Fermanagh, a territory which is called after them, Oriel.
Laigen territorial boundary (within the boundary of modern Leinster province) included the powerful Southern Uí Néill septs of Clann Cholmaín and Síl nÁedo Sláine of Mide and Brega.
The Airghialla federation included Uí Cremthainn, Uí Méith, Uí Tuirtre, Uí Meic Uais, Uí Fiachra Ard Sratha, Mughdorna, Uí Meic Cáirthinn, Airthir, Fernmhaighe, and Fir Lí, among other terms descriptive of tribal groupings and territories. The Uí Fiachrach descended from Fiachra, brother of Nial of the Nine Hostages, ancestor of the Uí Niell. Fiachra’s son and grandson were both High Kings in the second half of the fifth century, though after that the High Kingship of Tara was vested in the Uí Neill...
The North Gaels divided into two great branches; the tribal leadership of the Connachta itself divided into three. The Northern Uí Fiachrach and chiefs were known as Kings of the Moy, and branches of the family settled in Kerry before the end of the sixteenth century. The finest of all the Manks historic monuments is Castle Rushen. According to the Chronicle, Robert the Bruce laid siege to it in 1313, when Duncan Macdougall held it against him for more than three weeks. In the yeare 1164, Somerled assembled a fleet of 160 ships and put in at Renfrew, with the intention of subduing the whole of Scotland. In the yeare 1171, Richard, Earl of Pembroke, sailed to Ireland, and subdued Dublin, with a great part of Ireland.
The Book of Fenagh states the sub-territories of Airgialla were: Uí Nialláin (Oneilland baronies, co. Armagh.); Uí Bresail (now in the baronies of Oneilland); Uí Echach (baronies of Iveagh, co. Down); Uí Meith [Macha] (parishes of of Tullycorbet, Kilmore and Tehallan. co. Monaghan.); Uí Tortain (around Ardbraccan, co. Meath); Uí Briuin Archaill (in the barony of Dungannon, co. Tyrone); Trí Tuatha (here comprising Fir Lemna, Uí Cremthainne and Síl Dubthir of Lct.); Dartraige, alias Dartraige Coininnsi (barony of Dartry, c. Monaghan.); Fera Manach (Fermanagh), Fernmag (barony of Farney, co. Monaghan.); Mugdorn and Ross (Cremorne with the parish of Carrickmacross, and parish of Clonany, co. Monaghan., and adjoining parts of Louth and Meath undefined). Noted chiefs of Airghialla included Ua Laidhgnén (O'Leighnin?), Ua Éiccnigh (O'Heany or Hegney), Ua Cerbhaill (O'Carroll), Ua Baígelláin (O'Boylan), Ua Anluain (O'Hanlon), Mac Mathgamna (MacMahon), among others.
The Kings of Airghialla from Cremthainn, a fourth generation direct descendent of Colla da Crioch (see na tri Colla), to the times of the Lords of Fernmaig (see Clan Nadsluaig). Eochaid (or Echu), son of Crimthann (Uí Chrimthainn) is mentioned in the Tripartite Life of Saint Patrick as king of Lemhain, the Clogher Valley in County Tyrone. He was a contemporary of Saint Patrick.