Airgialla (Oriel)
The ancient kingdom of Airgialla was formed around AD 330. At one time, it included the southern parts of the modern counties of Tyrone and Derry, as well as much of Armagh, Monaghan and Fermanagh. With its royal site at Clogher, it included the

• Ui Thuitre
• Ui Cremthainn
• Ui Meith
• Airthir (Regional Kings of Cruachan (Connacht)
• Mugdorna
• Dairtre
• Fir Rois tribes

  Later the septs of the MacMahony, O'Hanlon, and Ó Neill of the Fews were prominent in this area. The Anglo-Norman advance in the 13th century broke up Oriel, but Monaghan remained dominated by the MacMahons and lay outside the main area of Anglo-Norman influence. The MacMahons maintained their national independence to the reign of Elizabeth; when Monaghan was formed into a county, and so called from its Chief town Muineachan, which signifies the "Town of the Monks." In 1589 a large area came under the English crown; in 1591 Monaghan was divided into estates between seven MacMahons and a McKenna and was not included in the later plantation of Ulster Lords of Oriel