O'Neill (Ó Néill), Domhnall (d. 1325), king of Tyrone, son of Brian O'Neill (qv) (d. 1260), the disputed high-king of Ireland, seems to have been little more than a boy when his father was killed at the battle of Down on 16 May 1260. His claims to the kingship of Tyrone were eclipsed by his cousins Aed Buide O'Neill (qv) (d. 1283) and Niall Cúlánach O'Neill (qv) (d. 1291). Nothing is known of Domhnall's early life, except that he married Gormflaith, daughter of Domnall Óc O'Donnell (qv) (d. 1281) of Donegal with whom he had at least five sons. O'Donnell's invasion of Tyrone in 1281 may have been an act of support for Domhnall's claims to the kingship; he was defeated and killed at Disertcreaght, 10 miles north of Dungannon, by Aed Buide and Thomas de Mandeville. Two years later Aed Buide died in battle, and Domhnall seems to have seized the kingship. There followed a long struggle to retain his position, which was marked by many reverses in his fortunes.
Domhnall is first mentioned in the annals in 1286, when he was deposed by Richard de Burgh (qv) (d. 1326), 2nd earl of Ulster and lord of Connacht, who banished him and installed Niall Cúlánach in his place. Four years went by, during which Domhnall gathered an army; in 1290 he invaded Tyrone and deposed his rival, who fled to de Burgh and invoked his support. The following year de Burgh came north, ousted Domhnall, and reinstalled Niall Cúlánach as king. Domhnall finally rid himself of his rival by killing him later in the year, but de Burgh with his army swept into Tyrone, forced Domhnall out, and set up as king of Tyrone another candidate of his own. According to the Annals of Loch Cé, the new king was Aed's Buide's son ‘Niall’, but the other annals agree that de Burgh made Brian II O'Neill (qv) (d. 1295), son of Aed Buide O'Neill, king of Tyrone. Whoever was Domhnall's immediate successor, Brian II was king by the close of the year, but he was heavily reliant upon the support of the earl and of the English of Ulster. In exile, Domhnall was gathering fresh forces to mount a renewed challenge for the kingship, and by 1292, with the help of his brother Niall O'Neill, he was rebuilding his power in Tyrone; in that year he killed Somhairle O'Gormley (Ó Gairmledaig). In 1295 Domhnall took action to reclaim Tyrone: at the battle of Creeve (Cráeb) he destroyed the combined forces of Brian II O'Neill and the English of Ulster, killing his rival. Thereafter his hold on the kingship was secure for a time, in spite of Burke's opposition.
This period of Domhnall's supremacy in the early thirteenth century coincides with a notable absence of references to unrest among the O'Neills in the annals; only one significant exception occurs – the killing in 1306 of Domhnall Tuirtreach O'Neill by Domhnall's household troops. After 1310 de Burgh renewed his attempts to reduce Domhnall's power over the Irish of central Ulster. In a charter of 1312–13 Diarmait O'Kane (Ó Catháin) acknowledged that he held his lands at Glenconkeine, Co. Derry, from de Burgh, rather than from Domhnall; in a second charter the earl then gave these lands to Domhnall's enemies the Clann Aodha Buidhe O'Neills. Under Brian II's son Henry O'Neill (qv) (d. 1347), this branch of the O'Neill dynasty were determined to wrest back Tyrone; their opposition, combined with de Burgh's extensive colonisation of the Derry coast and the Inishowen peninsula, threatened Domhnall and increased his hostility. By 1314 he was in regular contact with Robert Bruce, king of Scots, whom he may have sheltered and protected during his earlier exile in Ulster. After Bruce's victory at Bannockburn on 24 June 1314, Domhnall seems to have pressed him to dispatch an expeditionary force to Ulster, which Bruce did, under the command of his brother Edward Bruce (qv), whose attention he may have been all too happy to divert to Ulster.
On 25 May 1315 Edward landed with his army at Larne, where he was quickly joined by Domhnall and other Ulster kings. On Domhnall's advice, the army retreated to the north towards Inishowen, scorching the countryside as they went and destroying the bridge at Coleraine. De Burgh pursued them and on 10 September they turned and fought him at Connor, Co. Antrim, gaining a great victory and forcing the earl to flee to Connacht. Domhnall accompanied Edward's expedition into Leinster in early 1316; its success seems to have prompted Domhnall to support Edward's plan to claim the dormant high-kingship, and to shelve his own pretensions to the title. Edward had been crowned high-king of Ireland in May 1315 on the hill of Faughart, near Dundalk, though his position was eroded in the following year after the failure of expeditions into Leinster and Munster between February and May. However, these setbacks were insufficient to affect the control that he and Domhnall exercised over Ulster. Domhnall's status and ambitions are indicated by the famous remonstrance of the Irish to the pope, which he compiled in summer 1317. It styles him ‘king of Ulster and the true heir by hereditary right of all Ireland’ (Walter Bower, Scotichronicon, vi, 384–5; Curtis and McDowell, Irish historical documents, 38); it complains of the cruelty of the English to the Irish and the brutal murders of several Irish leaders by Englishmen; and it explains that Domhnall had called upon Edward for help, granting him his rights in the kingdom by letters patent.
In the autumn of that year, with the Lacys and Edward, Domhnall marched into Louth; at the hill of Faughart they were cornered by the English and heavily defeated, Edward was killed, and Domhnall fled. Later in the year Domhnall's son John and his ally Alexander MacDonnell (Mac Domhnaill) of the Isles were killed by Áed O'Donnell (qv) (d. 1333) at Derry. In early 1319 the English, with Henry O'Neill and the Clann Aodha Buidhe O'Neills, expelled Domhnall from Tyrone to Fermanagh; during the rout the Clann Aodha Buidhe O'Neills and Henry MacDavill (de Mandeville) killed another son of Domhnall, Brian, at Maghera in Derry. While Flaithbertach Maguire (Mág Uidhir) (qv) (d. 1327) of Fermanagh welcomed Domhnall and his defeated hosts into his kingdom, the people rose up and plundered the refugees. With his fortunes at so low a point, Domhnall made a last bid for power, returning to Tyrone and seizing back the kingship. He was never to lose it again, and remained secure in his leadership until his death. He died in 1325 on a crannóg in Loch Laoghair in the Tyrone barony of Clogher.