Oriel
Oriel was the remnant of Airgíalla, a kingdom in central Ulster. After the Anglo-Norman invasion removed its eastern lands, the MacMahons ruled the reduced kingdom around Monaghan from 1243.
In 1585, caught between the Tudor state and Tyrone's O'Neills, Rossa Boy MacMahon surrendered the lordship to the crown for regrant. It became County Monaghan, divided into five baronies. The MacMahons joined the Irish rebellion of 1641; after its defeat their lands passed to Protestant settlers.