Mac Lochlainn, Muirchertach (d. 1166), son of Niall, king of Ailech and high-king of Ireland, belonged to the dynasty of Cenél nÉogain. His father Niall, king of Tír Chonaill, was slain at the age of 28 just before Christmas 1119 in an intra-dynastic squabble within Cenél nÉogain. His mother, Caillech Chrín, was a daughter of a relatively obscure dynast named Ua Cuilén. His grandfather Domnall Ua Lochlainn (qv), king of Ailech, had also claimed the high-kingship of Ireland. There is no record of his marriage(s), but he had at least four sons: Conchobar (d. 1170), Niall (d. 1176), Máel-Sechlainn (d. 1185), and Muirchertach (slain 1196).
In 1136 Muirchertach son of Niall, aged probably in his mid twenties, laid claim to the kingship of Ailech following the death of his uncle, Conchobar. His hold on the kingship in those early years, however, was far from secure and it seems that he was not firmly established till 1145. In that year, he managed to depose a rival claimant – Domnall Ua Gairmledaig, who belonged to a minor branch of Cenél nÉogain. Gaining in confidence, he invaded the overkingdom of Ulaid in 1147, with the support of Donnchad Ua Cerbaill (qv) king of Airgialla, taking rich spoils and a number of valuable hostages. He subsequently partitioned Ulaid between four Dál Fiatach dynasts, but when this initiative threatened to destabilise his relationship with Ua Cerbaill, he chose to reverse the plan and restore Cú Ulad Mac Duinn Shlébe as regional ruler. Clearly the strongest figure in the northern provinces by the late 1140s, he contested with the powerful overking of Connacht, Tairdelbach Ua Conchobair (qv), for the high-kingship of Ireland. In 1149 he made a great hosting to Louth and subsequently proceeded to Dublin. On the way, he received submissions from several regional kings including Tigernán Ua Ruairc (qv) of Bréifne, Murchad Ua Máelshechlainn (qv) of Mide, and Diarmait Mac Murchada (qv) of Leinster.
This apparently meteoric rise of Cenél nÉogain overawed the ageing Ua Conchobair, who sought to make terms in 1150. An agreement was reached, whereby the politically weak overkingdom of Mide – the strategic value of which was recognised by all ambitious provincial rulers – was to be divided between Muirchertach's allies, Ua Cerbaill and Ua Ruairc, with the western part reserved for Ua Conchobair himself. At this point Tairdelbach Ua Briain (qv) king of Thomond, presumably concerned about this expansion of northern influence into the midlands, sought to intervene in defence of Ua Máelshechlainn's interests. The result was that Ua Conchobair and his allies invaded northern Munster in 1151, crushed Ua Briain at Móin Mór (Mourneabbey, Co. Cork), and assuaged Cenél nÉogain sensitivities by tendering a formal submission.
Muirchertach certainly assumed a supremacy on his own part, and strove to make his political imprint on the important synod of Kells–Mellifont of 1152, which reorganised Ireland's diocesan structure. Following a royal conference at Beleek that same year, it was decided to restore Ua Máelshechlainn and his son jointly to the kingship of Mide. It seems, however, that Ua Ruairc objected to this new arrangement, and Muirchertach, with Ua Conchobair and Mac Murchada, invaded Bréifne and burned the Ua Ruairc fortress at Daingen; it was in the context of these events that the so-called ‘abduction’ (or, perhaps, flight) of Derbfhorgaill (qv) queen of Bréifne took place. Such collaborative actions notwithstanding, it soon emerged that Ua Conchobair had not abandoned his own claims to supremacy, at least within the southern half of Ireland. When the latter again marched into Munster in 1153, Muirchertach went on the offensive, driving him back into Connacht. He then reinforced Mide as a bulwark against Connacht expansion in the midlands, enlarging its boundaries to include the north Leinster sub-kingdoms of Uí Fhailge and Uí Fháeláin. He also restored Ua Briain of Thomond and Ua Ruairc of Bréifne – both of whom had more reason to serve his interests than those of Ua Conchobair.
Despite the apparent success of his political initiatives, Muirchertach realised that he could not consolidate his hold on the midlands unchallenged by Connacht. In 1154, in response to a naval offensive from that quarter, Cenél nÉogain hired a fleet from the kingdom of Man and the Isles, only to see it devastated by Ua Conchobair's ships. By way of response, Muirchertach invaded Connacht and destroyed a royal fortress at Dún Imdáin (Dunamon, Co. Roscommon). He then marched on Dublin, where he received the submission of the Hiberno-Norsemen and granted them an impressive stipend of 1,500 cattle – an implicit warning to would-be challengers. The following year, when Máel-Sechlainn Ua Máelshechlainn died, he took the opportunity to install his own candidate in Mide. For all his efforts, though, the elderly Ua Conchobair remained undeterred and was, in 1156, assembling a coalition to invade Cenél nÉogain when he died suddenly.
Muirchertach was now the dominant political force in the country; styling himself rex totius Hiberniae (king of all Ireland), in 1157 he endowed the new Cistercian foundation of Newry, witnessed by the highest ecclesiastical dignitary in the land, Gilla Meic Liac (qv), archbishop of Armagh. The following year, he provided the driving force behind the synod of Brí meic Thaidc (in Co. Meath), which established the authority of the abbot of Derry, Flaithbertach Ua Brolcháin (qv), over the Columban federation. Later, that authority was more clearly defined when Columban churches in Meath and in Leinster were declared to be free from lay taxation.
Meanwhile, Muirchertach made a ‘great circuit’ of Ireland to receive renewed submissions from Leinster and Munster. He still faced challenges to his supremacy: during his absence on circuit, Ruaidrí Ua Conchobair (qv), who had succeeded his father as overking of Connacht, led a raid into Cenél nÉogain. In 1158, when Muirchertach was preoccupied with problems closer to home, Ruaidrí plundered Mide. Although Muirchertach successfully countered this challenge, defeating Ruaidrí (to whose side Ua Ruairc had now defected) at Ardee in 1159, he did not manage to reduce the province of Connacht. Instead, in 1161 he made a deal with his opponents, whereby the Ua Conchobair interest gained control of western Mide. He then subdued Brega and overawed Dublin, making the town directly obedient to himself and to his leading tributary king, Mac Murchada.
In the years that followed Muirchertach was at the height of his power. In the perhaps partisan Annals of Ulster he is styled rí Érenn cen fressabra (king of Ireland without opposition). The security of tenure implicit in this claim, however, was never fully realised. Already, even in northern Ireland, opposition to his suzerainty was beginning to emerge. In 1164 he managed to suppress a rebellion within Cenél nÉogain without too much difficulty; but his handling of an Ulaid revolt against his overlordship the following year led to his downfall. The Dál Fiatach king of Ulaid, Eochaid Mac Duinn Shlébe, had fled on the advance of his army – only to be captured and imprisoned by Ua Cerbaill. At this point, Muirchertach promised faithfully to restore him as a tributary king, swearing on the Bachall Ísu – an important Patrician relic – in the presence of Archbishop Gilla Meic Liac, Ua Cerbaill and others. His action, despite this solemn guarantee, in seizing and blinding the hapless Mac Duinn Shlébe at Easter 1166, caused widespread revulsion and fanned the embers of revolt throughout the north.
As he struggled to retain control, with Ua Cerbaill and other erstwhile allies now ranged against him, Ua Conchobair and Ua Ruairc swept through the midlands and promoted a rebellion in Leinster that reduced his principal ally Mac Murchada to fighting for his life. By the summer of 1166, with his enemies closing in on mid-Ulster and the leader of the main rival dynastic line Áed Ua Néill laying claim to the kingship of Cenél nÉogain, Muirchertach's authority was rapidly disintegrating. Still striving to resist, but with a dwindling band of supporters, he fell back towards the fringes of his realm – only to be slain and decapitated by the Airgialla in a minor engagement in Fid Ua nEchdach (seemingly in Co. Armagh). Despite initial resistance from some senior clergy because of the forswearing of the solemn oath, Gilla Meic Liac admitted Muirchertach's body into the cathedral for funeral services, and he was interred in the royal cemetery. The high-kingship that he had striven so hard to hold was assumed by Ua Conchobair, only to be undermined by the Anglo-Norman conquests of the 1170s. Muirchertach's sons held the kingship of Cenél nÉogain in turn till 1196, when the younger Muirchertach fell in an internal conflict and political dominance passed to the Ua Néill line in the person of Áed Méith (qv) (d. 1230).