The MacMahons of Oriel are a sept, or sub-clan of Clann Nadsluaigh. They trace descent from The Three Collas to their patronymic namesake, Niall Mac Mathghamhna, who descended from Mathgamna, Lord of the ancient kingdom of Fernmaige, located in the modern Baroney of Farney. They ruled the kingdom of Oriel (Anglicisation of Airgíalla) from 1243 to 1590 C.E. The Airgíalla Kingship was ended when Rossa Buidhe MacMahon, found himself in a geopolitically untenable position of his kingdom wedged between an expanding Tudor Kingdom of Ireland and Tír Eoghain under the O'Neill. Rossa Buidhe agreed to surrender and regrant his territories to the English Crown in Ireland and they became County Monaghan in the Kingdom of Ireland. The county was subdivided into five baronies with Farney, Cremorne, Dartrey, and Monaghan controlled by MacMahons and Truagh by McKennas. The MacMahons lost control of Monaghan after the Irish Rebellion of 1641. Led by Colla Mac O'brian MacMahon, the MacMahons rose in revolt with O'Neill and other Ulster clans in an attempt to seize Dublin Castle and overthrow the English Tudor kingdom. While the plot to capture Dublin Castle failed, the McMahons captured Castleblaney and Carrickmacross in Monaghan. However, the English defeated the Irish uprising and Monaghan land was passed to Protestant settlers. It was a MacMahon, Colonel Hugh Oge's loose lips that were responsible for letting slip the plot to seize Dublin Castle. He had drunkenly told his Protestant foster brother Owen Connolly, who subsequently betrayed him and he was captured. With the help of two priests and a Scottish maid, Hugh Oge escaped from the Tower of London by sawing through his bars on 18 August 1644. However, he was recaptured 5 weeks later. He was convicted of treason and beheaded in London on 22 November 1644. John O'Hart also noted that the early MacMahons (sometimes called O'Mahons) were chiefs of the over-kingdom of Ulaid, which bordered Airgíalla. Chieftains Neill mac Mathgamhna mac Mathgamna ?-? (Ladrannaibh, or the bandit), (early 12th century) Eochaid mac Mathgahamna mac Neill, died 1273 Brian mac Eochada, 1283–1311 Ralph/Roolb mac Eochada, 1311–1314 Mael Sechlainn mac Eochada, 1314–? Murchad Mór mac Briain, ?–1331 Seoan mac Maoilsheachlainn, 1331–1342 Aodh mac Roolb, 1342–1344 Murchadh Óg mac Murchada, 1344–1344 Maghnus mac Eochadha, 1344–1357 Pilib mac Rooilbh, 1357–1362 Brian Mór mac Aodh, 1362–1365 Niall mac Murchadha, 1365–1368 Brian Mór mac Aodh, 1368–1371 Pilib Ruadh mac Briain, 1371–1403 Ardghal mac Briain, 1403–February 1416