O'Donnell (Ó Domnaill), Domnall Óc (1241–81), was posthumous son of Domnall Mór O'Donnell (qv), king of Tír Conaill, and Laisirfina, daughter of Cathal Mór Crobderg Ua Conchobair (qv), king of Connacht. At 18 he was catapulted to the kingship of Tír Conaill by the death of his heroic brother Gofraid O'Donnell (qv), king 1248–58. Like his elder brother, Domnall Óc was faced by the twin threats of Brian O'Neill (qv) of Tír Eógain and Maurice FitzGerald (qv) to the relatively recent O'Donnell governance of Tír Conaill. Despite his complete rout by the dying Gofraid at Lough Swilly, O'Neill judged the time was ripe in 1258 to dispatch messengers to the young king, demanding his submission. Domnall Óc, of course, ignored O'Neill's arrogance. In 1259, however, he gave O'Neill his answer by linking up with Áed Buide O'Neill (qv) to burn Tír Eógain. By the next year he was back on the defensive, fighting off the intentions of FitzGerald and burning his Cairbre lands in Sligo. But his continuation of his father's alliance with his O'Connor cousins and the defeat of O'Neill at Down (16 May 1260) allowed him to develop his power in the north-west and extend it into parts of Tír Eógain itself; and in 1262 he campaigned as far as the Meath territory of Teffia. Indeed, much of his military strength may have been ensured by the fact that he, like his cousin Áed mac Eógain O'Connor (qv), married wives successively from two powerful Scottish mercenary families, the MacSweeneys and the MacDonnells. In 1263 he and Áed pooled their forces and devastated the colonists of Connacht, burning as far as Galway. Also that year Domnall Óc is separately mentioned as having treacherously killed Donn O'Breslin (O'Breslein), chief of Fanad, at the bishop's court at Raphoe. Again on the death (1265) of Áed's father Fedlimid (qv), king of Connacht, Domnall Óc and his cousin levelled the strategic FitzGerald fortress of Sligo. This drew a sharp response in the form of a campaign being led into Tír Conaill by Walter de Burgh (qv), earl of Ulster and lord of Connacht, and Áed Buide O'Neill. However, Domnall Óc was strong enough to resist this combined force and marched into Sligo to rendezvous with Áed, burning several castles and settlements. For the next few years he remained at peace but returned to war in 1270, razing Sligo again. With the aid of his O'Connor cousins, he furthered his advance into Ulster at the expense of the traditional O'Neill over-kingship. During 1272 he placed a fleet on Lough Oughter in Cavan and exploited O'Neill weakness to plunder Tír Eógain during the following year as well as routing the O'Neill invasion of Tír Conaill (1275). Two years later he was back in Connacht, aiding Áed Muimnech O'Connor to destroy Roscommon castle. And because of continuing O'Neill weakness and the not inconsiderable dynastic troubles of the O'Connors of Connacht, Domnall Óc emerged from the shadows to establish himself as the most powerful Irish king in Ulster and Connacht and thus the greatest O'Donnell yet. However, the annalistic accolades as ‘king of the North’ were brief, as during a 1281 invasion of Tír Eógain he was surprisingly defeated and killed at Dísert da Chrích (Desertcreaght, barony of Dungannon) by Áed Buide O'Neill and the English of Ulster.
His body was interred at Derry and he was succeeded by his son, Áed (d. 1333). Nothing is mentioned of the first four years of Áed's reign, but his position may have been compromised by a struggle with his younger half-brother, Toirrdelbach (d. 1303). Perhaps this feud allowed de Burgh to exert himself over Tír Conaill through his taking of Áed's hostages (1286). In 1290 Toirrdelbach made his move, overthrowing Áed with the aid of his MacDonnell galloglass kinsmen. However, Toirrdelbach was allowed precious little time to consolidate his position, as de Burgh pounced and forced his submission in 1291. Despite the vigorous efforts of his ally Magnus O'Connor, king of Connacht, Toirrdelbach was unable to resist the rebellion fomented by Áed, who promptly expelled his sibling. Although Toirrdelbach was down he was not out, and a prolonged civil war ensued, ending in his death at the hands of Áed (1303). Indeed, one source mentions that Áed never had a bad day as king of Tír Conaill after his killing of Toirrdelbach. He seems to have strengthened his position by marrying Derbforgaill (d. 1316), a daughter of Magnus O'Connor. In 1315 Áed, at the insistence of Derbforgaill, destroyed Sligo castle and even allowed his wife to lead her Clann Muirchertaig O'Connor kinsmen and a force of galloglass to devastate the wider Sligo region. For the most part his later reign was peaceful, although he reputedly killed Seoán, son of Domhnall O'Neill (qv), king of Tír Eógain, at Derry (1318) and was forced to expel some of Toirrdelbach's sons. But such was the strength of his grip on Tír Conaill that he entered monastic orders and passed the kingship on his death (1333) to his son, Conchobar (qv).