The Mac Mahon (Mac Mathgamna) family were Gaelic lords of Airgialla (Oriel), approximating to what is now County. Monaghan, during the late medieval period, on the marchlands of Gaelic Ulster and the northern Pale. They are first mentioned in the annals in 1181, and were related to the previous Ua Cerbaill ruling dynasty of Airgialla who surrendered the lordship to the Mac Mahons in the late twelfth century. The Mac Mahons were a border people, who came alternately under pressure from Ua Neill, lord of Tfr nEogain, and the Anglo-Irish of Louth. Throughout the entire medieval period the Mac Mahon lordship was also a relatively poor region. There were no important religious houses or Mac Mahon castles, although during the fifteenth century the Mac Mahon lords did become noted patrons of bardic poets.
The first powerful Mac Mahon ruler was Niall, who ruled Airgialla in the late twelfth and early thirteenth centuries. Niall employed mercenary bandits and moved the Mac Mahons from Farney into Cremorne. He fought both English and Irish enemies, being an ally of John de Courcy. His most notable exploit was the killing of Eicnecan Ua Domnaill, lord of Tfr Conaill in 1207, who was killed while on a raid into Fermanagh. The Mac Mahons came under severe pressure from the English of Louth in the 1250s and 1260s, which greatly weakened the family. They then came under the overlordship of the earls of Ulster and the de Verdons of Louth, who imposed claims for military service on them. As the fourteenth century progressed, the dynasty became more powerful, and by the end of that century various branches of the Mac Mahon family ruled the territories of Farney, Cremorne, Dartry, and Monaghan, with the Mac Kenna family of Truagh under the overlordship of the ruling Mac Mahon. The mid-fourteenth-century lord, Brian Mor, was the most powerful ruler of Airgialla. In 1346, he won a great victory over the English of Louth when he killed over 300 English soldiers. In 1365, Brian "assumed the lordship of Airgialla," but became embroiled in a war with Ua Neill of Tfr nEogain for drowning Somhairle MacDomnaill, Ua Neill’s galloglass constable. Brian Mor’s chief fortress was at Rath-Tulach in the barony of Monaghan. He was slain in 1372 by one of his own bodyguards. Brian Mor levied black rents on the English of Louth, which included tributes of fine clothes, silver, and malt, and was referred to as "undisputed high king of Airgialla, a man who held sway [over the territory extending] from the Boyne to Derry, and from Gleann righe [Newry] to Bearamhan in Breifne." However, in 1368, even Brian Mor had to yield half of Airgialla to Niall Mor Ua Neill as eiric (payment; compensation; legal fine, especially for violation of honour or manslaughter) for killing Ua Neill’s gal-loglass constable, and in 1370 Ua Neill killed "very great numbers of Mac Mahon’s people." In the late fifteenth century, Aed Oc (1485-1496) was a powerful figure. In 1486, he burned 28 townlands belonging to the English in Airgialla and in 1494 inflicted a sharp defeat on an English force, killing 60 gentlemen. Having become blind, he died in 1496. His brother Magnus was noted for displaying the severed heads of his English enemies on the palisade around his bawn at Lurgan.
By the sixteenth century there were three main branches of the Mac Mahon family in Airgialla, the branches of Monaghan, Dartry, and Farney, all descended from Ruaidrf who died in 1446. From 1513, the Monaghan branch of the dynasty monopolized the chieftaincy. English interference in the Mac Mahon lordship became very serious as the sixteenth century progressed. In 1576, Walter Devereux, earl of Essex, was granted the barony of Farney, and in 1590, the Mac Mahon chieftain, Hugh Roe, was executed by Lord Deputy William Fitzwilliam, who divided Air-gialla up among the chief lords and freeholders. During the Nine Years’ War, Brian Mac Hugh Og Mac Mahon of the Dartry branch, was a prominent leader, of note for betraying the confederate cause on the eve of the battle of Kinsale for a bottle of whiskey.
It is important to note that during the medieval period there was an important and completely separate Mac Mahon family, lords of Corcu-Baiscinn, in Thomond, descended from the high king, Muirchertach Ua Briain (d. 1119). In 1404, they are referred to as Mac Carthy’s "chief maritime officer." The chieftain, Tadhg Caech Mac Mahon, lord of west Corc-Baiscinn, (1595-1602), was prominent during the Nine Years’ War on the Confederate side. He was expelled from his lordship by the earl of Thomond and fled to Red Hugh O’Donnell, the lord of Tfr Conaill. Tadhg Caech was shot dead in 1602, it being stated that "There was no triocha-chead in Ireland of which this Tadhg was not worthy to have been lord, for [dexterity of] hand, for bounteousness, for purchase of wine, horses, and literary works."
