Ua Lochlainn (Mac Lochlainn), Domnall (1048/9–1121), son of Ardgar and overking of Leth Cuinn (the northern half of Ireland), was an early representative of the Mac Lochlainn family line within the Northern Uí Néill dynasty of Cenél nÉogain. Domnall's father Ardgar (d. 1064) had held the Northern Uí Néill overkingship of Ailech. The identity of his grandfather has been a matter of some dispute, but the Lochlainn in question was probably a son of Máel-Sechnaill (d. 997); a verse in the Annals of Ulster at 1099 describes Domnall as a descendant of Flann, and the grandfather of Máel-Sechnaill was named Flann. Domnall's mother was Órlaith great granddaughter of Máel-Caindig – probably a minor north-western dynast. Domnall himself was married to Bébinn (d. 1110), a daughter of Cennétig Ua Briain, a bitter rival of the powerful Munster overking Tairdelbach Ua Briain (qv) (d. 1086), claimant to the high-kingship of Ireland. His sons included Conchobar and Niall; it seems there was another son (whose name in the Annals of Inisfallen is partly obliterated but should perhaps be read as Fergal). He also had a daughter, Mór (d. 1122), who became the wife of Tairdelbach Ua Conchobair (qv), overking of Connacht.
From 1083 Domnall emerged as a leading dynast of the Uí Néill; in that year he took the kingship of Cenél nÉogain, and immediately led a ‘king's raid’ on the territory of Conaille (north Co. Louth), distributing the booty as stipends to his retainers. After two decades of unrest within Cenél nÉogain, Domnall inherited a kingship severely weakened by conflict, which had already attracted the attention of Tairdelbach Ua Briain. Within a year of his accession, he successfully faced a challenge from the Ulaid, under their king Donn Slébe. In the decades that followed, he strove to maintain supremacy in the north; with varying degrees of success he drew Connacht and the midlands into conflict with Munster, as he sought to challenge the claims of Muirchertach Ua Briain (qv) to the high-kingship of Ireland. In 1088, after the collapse of negotiations with Ua Briain, Domnall first subdued Connacht, forced its overking Ruaidrí na Saide Buide Ua Conchobair (qv) – who was married to a daughter of Ua Briain – into an alliance, and invaded Munster. Domnall and his ally burned Limerick, sacked Kincora, devastated much of northern Munster, and seized hostages. Taking advantage of continuing political dissension amongst the Ua Briain rulers, Domnall (now styled ‘king of Ailech’) convened a rígdál (royal meeting) in 1090, as a result of which he again took hostages of Munster and of Meath. Meanwhile, he proceeded to consolidate his authority within the north; in the first seven years of his kingship he slew at least four minor dynasts of the Northern Uí Néill. In 1091 he slew Donn Slébe, overking of Ulaid, in battle near Newry and subdued that realm. Two years later, as a challenge again emerged from the south, Domnall first reasserted his authority over the north-western kingdom of Cenél Conaill and then convened another rígdál, at Dornann Dabhaill on the shore of Lough Neagh (probably near Maghery, Co. Armagh), to secure the support of the Ulaid. Crossing through Meath, he engaged Ua Briain's forces in north Co. Kildare, but then had to abandon his initiative as the Ulaid contingent refused to proceed with an invasion of Leinster, with whose rulers they had formerly had an alliance. Later, Domnall punished the Ulaid for their failure to support him. In 1099 he invaded their kingdom, cut down their inauguration tree at Cráeb Tulcha (Crew Hill, parish of Glenavy, Co. Antrim) and took hostages; the following year he captured Donnchad Ua hEochada (qv), overking of Ulaid, with several of his nobles, and held him prisoner for a year.
Although he had been deprived of an opportunity to strike against Ua Briain's interests, Domnall certainly benefited from his relationship with the church when faced with a military threat from the direction of Munster. On three successive occasions (1097, 1099, 1102) he was spared an invasion of the north when the abbot of Armagh, Domnall (qv) son of Amalgaid, intervened on his behalf. Battle was joined with the forces of the south, however, in 1103. This particular conflict arose from yet another incursion by Domnall across the Bann, in response to which Ua Briain took the side of the Ulaid. Seizing his opportunity when an invading Munster force camped at Armagh, and its main force moved on to plunder the territory around Lough Neagh, on 5 August 1103 Domnall descended upon the enemy rearguard at Mag Coba (near Dromore, Co. Down), inflicting great slaughter. He then proceeded not only to take the hostages of the Ulaid, but to assert his authority over Meath. Later (1106) he helped the king of Meath to enforce his authority in the west of that province. There were three further occasions on which the Munstermen attempted to invade the north, but in 1105 the elderly abbot of Armagh intervened on Domnall's behalf one last time; then the new abbot, Cellach (qv), secured peace between the protagonists in 1107, 1109, and again in 1113.
Meanwhile, in 1110 Domnall managed to defeat an invading force from the south at Es Ruaid (Assaroe, on the River Erne), and to beat off a naval attack on the Inishowen peninsula. A year later, Domnall's caput at Ailech was destroyed by Ua Briain, who went on to plunder the territory of Cenél nÉogain and to take the hostages of the Ulaid. The discomfiture of Domnall, however temporary, encouraged the Ulaid to make an incursion into Cenél nÉogain, where they cut down the sacred trees of Telach Óc (Tullaghogue, Co. Tyrone). In revenge, Domnall's son Niall carried out a cattle raid against the Ulaid. In any event, Domnall recovered quickly from his setback. In 1112 he plundered northern Connacht, reasserted his authority over the Ulaid, and plundered Meath and Fine Gall up to the walls of Dublin. In a bold move that same year, he appointed his 21-year-old son Niall as king over Cenél Conaill – a position to which he had no entitlement under established laws of regnal succession. Encouraged, perhaps, by the apparent success of this move, in 1113 Domnall dealt summarily with the kingdom of Ulaid, which had frustrated his designs on several occasions. He deposed its overking, Donnchad Ua hEochada, partitioned the core-kingdom of Dál Fiatach, and brought the two principal sub-kingdoms directly under his own rule. Domnall's assumption of the role of dominus terre in this regard certainly appears innovative. In 1114 Domnall felt ready to strike again at Munster. He led the hosts of the north through Meath and Connacht, where he secured the support of the new overking, his son-in-law Tairdelbach Ua Conchobair. The combined forces marched to the border of Dál Cais, where Ua Briain made terms.
In the years that followed, Domnall's scheme for mastery of Leth Cuinn commenced to unravel somewhat. His son Niall was slain, aged 28, just ten days before Christmas 1119 by a disaffected segment of Cenél nÉogain. Domnall had by this time become increasingly concerned at the growing power of Connacht and with its efforts to dominate both Meath and Munster. Conscious, nonetheless, that Ua Conchobair had observed the spirit of his marriage alliance and had avoided conflict with Leth Cuinn, Domnall marched to Athlone in 1120 and they made terms. This was his last recorded action; he died at Derry on Wednesday 10 February 1121, in his seventy-third year. He is praised by some annalists not only for his personal bravery, but for his wisdom and generosity. Domnall's immediate successor in the kingship of Cenél nÉogain was his son Conchobar, who was not as strong a ruler as his father had been. Conchobar died in 1136; a brother, whose name was perhaps Fergal, was slain seven years earlier by the Ulaid. Niall, who as already noted predeceased his father, left a young son, Muirchertach Mac Lochlainn (qv), who later enjoyed an illustrious career and strongly asserted the claims of his family line to the high-kingship of Ireland.