Mac Gilla Pátraic (MacGillapatrick), Domnall (d. 1185), king of North Osraige (Ossory), was probably the eldest son of Donnchad Mac Gilla Pátraic (d. 1162), king of Osraige. When Donnchad died the kingdom was divided into northern and southern portions; North Osraige passed to Domnall son of Gilla Pátraic Mac Gilla Pátraic, then, when he was killed in battle against the Uí Mordha (O'Mores) of Laois in 1165, to Domnall, while the southern part was ruled over by Domnall (d. 1176), son of Cerball Mac Gilla Pátraic. There was long-standing antagonism between the Mac Gilla Pátraic kings of Osraige and the Mac Murchada (MacMurrough) kings of Leinster, which had lately drawn fresh venom from the attempts of Diarmait Mac Murchada (qv) (d. 1171) to instal a member of his Uí Cháellaide foster family as a puppet king of Osraige. To counter his ambitions the Mac Gilla Pátraic allied themselves with his rival, Ruaidrí Ua Conchobair (qv) (d. 1198), the ambitious and able king of Connacht.
The opportunity to revenge themselves upon the Leinster king presented itself in 1166, when Mac Murchada's ally, high-king Muirchertach Mac Lochlainn (qv) (d. 1166), blinded Eochaid Mac Duinn Sléibe, king of Ulaid. Mac Lochlainn's action angered Donnchad Ua Cerbaill (qv), king of Oriel, Eochaid's foster father; he transferred his support to Ua Conchobair, who marched to Dublin to be crowned high-king of Ireland. Ua Conchobair invaded Mac Murchada's heartland of Uí Chennselaig, defeating him at the pass of Fid Dorcha – a victory that forced the submission of the Leinster king. The grateful Mac Gilla Pátraic gave hostages to Ruaidrí. However, Mac Murchada was determined to take revenge on them and the rebellious Leinstermen, who had betrayed him by supporting Ó Conchobair: while Ua Conchobair was hunting down Muirchertach Mac Lochlainn in Ulster, Mac Murchada encouraged Ó Braenáin (O'Brennan) to assassinate Mac Gilla Mo Cholmóc, the rebel king of Uí Briúin Chualann; but Mac Murchada's recovery in Leinster sparked another rebellion of the provincial nobility, along with the Ostmen of Dublin, forcing the Leinster king to take ship for England in early August. Before he fled he dealt the only blow he could to his persecutors by executing the hostages of the Mac Gilla Pátraic and a son of Fáelán Mac Fáeláin (qv) (d. 1203), king of Uí Fáeláin. Ua Conchobair rewarded the Mac Gilla Pátraic with half of Uí Chennselaig, granting the other half to Murchad Mac Murchada (qv) (d. 1172), Mac Murchada's rebel brother. Domnall also captured Mac Murchada's son Énna. At his convention in Athlone towards the end of 1166 Ua Conchobair presented the Mac Gilla Pátraic with twenty-five choice horses.
In 1167 the Mac Gilla Pátraic further demonstrated their attachment to the new high-king by joining his campaign to Armagh. At this stage their fortunes were riding high, but in August 1167 Mac Murchada returned and easily reasserted his power over Uí Chennselaig. Ua Conchobair prosecuted a successful winter campaign in Uí Chennselaig, but the Mac Gilla Pátraic remained apprehensive about Mac Murchada's intentions and in 1168 Domnall took the ill-judged decision to blind his hostage Énna. The vengeance of Mac Murchada and his challenge to Ó Conchobair's high-kingship began in May 1169 with the help of English mercenaries: he took Ostman Wexford and raided the clients of Ua Conchobair in west Leinster, rampaging into Osraige. The Mac Gilla Pátraic kingdom of South Osraige apparently collapsed, but in the more defensible landscape of North Osraige, Mac Gilla Pátraic resistance stiffened. Domnall chose his ground well – a pass between two densely forested hills – and improved his defences by constructing three fortified ditches across the breadth of the pass. After a day of fierce fighting Mac Murchada forced Domnall to retreat, but finding the going hard ordered a withdrawal of his own men to more hospitable territory; Domnall, lurking on Mac Murchada's flanks, immediately ordered his forces forward. Maurice de Prendergast (qv), realising their peril, urged Mac Murchada to exit the pass; the allies escaped into the plains and drew up their cavalry and archers to await Domnall. In his eagerness to gain the advantage, Domnall made the critical mistake of following the English into the open, exposing himself to a devastating cavalry charge, in which the Mac Gilla Pátraic were slaughtered. The battle was the first major confrontation between the contrasting styles of warfare of the Irish and Latin Europe.
Mac Murchada then turned north to deal with the rebellious Leinster nobility and in a brutal campaign pulverised the kingdoms of Uí Fáeláin and Uí Muiredaig, in the teeth of which the Ostman king of Dublin meekly submitted. In another confrontation Domnall forced Mac Murchada into a bloody struggle, lasting three days, at the fortified pass of Freshford, but eventually gave way before superior strength and was routed. This defeat showed that Domnall was becoming incapable of defending his kingdom, which Mac Murchada ransacked from end to end. But now fortune turned in Domnall's favour when a quarrel broke out between Mac Murchada and his English allies. Maurice de Prendergast wished to return with his men to Wales and to prevent him Mac Murchada dispatched riders to the port of Wexford to forbid any captain to transport them. In retaliation Prendergast offered an alliance to Domnall, to which the increasingly desperate king of North Osraige eagerly agreed, arranging a meeting at St Mullins in Carlow. Mac Murchada ordered his son, Domnall Cáemánach (qv) (d. 1175), to block the pass of Poulmounty, which lay on Prendergast's route to the tryst, but in spite of vehement resistance he was unable to stop Prendergast and his men, who after three days of marching and fighting arrived at St Mullins. Domnall and Prendergast entered into an alliance, taking oaths over the sacred shrine and relics never to betray each other.
Domnall's fortunes improved further when the high-king Ruaidrí Ua Conchobair determined to take action against Mac Murchada, perhaps at least in part to avert the imminent collapse of Domnall's kingdom. However, after some sparring with Mac Murchada in the forests of Uí Chennselaig, Ó Conchobair's confidence petered out and, avoiding a trial of arms, he dispatched clerics to treat with the Leinster king. A deal was struck, which confirmed Mac Murchada as king of Leinster in return for his recognition of Ó Conchobair's high-kingship; the English were to be sent home and Mac Murchada's last legitimate son, Conchobar, was taken hostage and promised one of Ua Conchobair's daughters in marriage. Believing that Mac Murchada had no intention of keeping his word, Domnall, aided by Prendergast, ravaged the north of Uí Chennselaig unremittingly, and then expanded his sphere of operations by attacking the Uí Mórda (O'Mores) of Laois, Mac Murchada's allies. Ó Mordha temporised with them, offering them terms and hostages, while sending for help to Mac Murchada, who gathered a large force, including an English contingent, and rode to the assistance of his ally. Warned by a scout of his approach, Prendergast and Domnall made a swift retreat to Osraige.
Shortly afterwards the men of Osraige, angered by Domnall's payment of high wages to Prendergast for his services, plotted to murder him and his men, and in an audience with the king informed him of their intention. When Prendergast got wind of the plot he requested to be allowed to return to Wales and Domnall agreed, though his ally's departure gravely depleted his ability to withstand Mac Murchada. By late August 1170 Mac Murchada's campaign was boosted by the arrival of two English fleets. On 23 August 1170 Richard fitz Gilbert de Clare (qv) (d. 1176), earl of Pembroke and Striguil, landed, and two days later Waterford fell to the English and Mac Murchada. Within a month Mac Murchada and de Clare had outmanoeuvred Ua Conchobair at Clondalkin and on 21 September they seized Dublin. The retreat of Ua Conchobair left Mac Murchada in control of virtually all east Leinster, and also removed the screen that protected his clients in west Leinster. The impact of the English cavalry was so devastating that it forced Domnall Mac Gilla Pátraic and Faelan Mac Fáeláin of Uí Fáeláin to flee for their lives to Connacht.
Domnall's exile was brief as Ua Conchobair seems to have encouraged him to return to Osraige to continue the fight. In early 1171 he reasserted himself over his devastated kingdom, enjoying a victory over Domnall Ó Focartaig (O'Fogarty), king of South Éle, killing him and 300 of his men. Success was fleeting, however. After the death of Mac Murchada in May 1171 the Leinster nobility rose up against his successors, Clare (his son-in-law) and Domnall Cáemánach Mac Murchada (his son), but in September the earl broke the blockade of Dublin by Ua Conchobair, the Ostmen, and the Leinstermen, allowing the English to regain the upper hand, and deciding Clare to deal once and for all with Domnall. Accordingly Clare appealed for support to his brother-in-law Domnall Mór Ua Briain (qv) (d. 1194), king of Thomond, and their forces converged in Idogh to the east of Mac Gilla Pátraic country. Realising that he was in a hopeless position, Domnall sent envoys, asking for a safe conduct so he might present his case to Clare. Prendergast, who had rejoined his countrymen, sent assurances to Domnall that he would guarantee his safety. At their meeting Clare accused Domnall of betraying Mac Murchada and, encouraged by Ua Briain, clearly intended to have Domnall executed. All seemed lost until Prendergast intervened, arguing that his honour was at stake as he had guaranteed Domnall's safe passage; his oratory succeeded in persuading Clare to release Domnall, though probably not before he had submitted to the earl and become his vassal.
Upon their return to Osraige, Domnall and Prendergast discovered that the troops of Ua Briain had plundered the land. Bereft of Ó Conchobair's protection, Domnall and many of the Leinstermen, acknowledging the political reality, welcomed the arrival in Ireland of Henry II (qv) of England in late autumn 1171, whose protection they sought; among the Irish princes who submitted to him were Domnall and Máel Sechnaill MacFáeláin of the Decies. Although this represented a capitulation, Domnall emerged relatively unscathed from his encounter with the king: the only part of North Osraige given to the English settlers belonged to the Uí Chaellaide, his bitter enemies, and in 1172 Domnall killed Ua Cáellaide, Mac Murchada's former protégé as king of Osraige.
In 1173 Domnall with many of the Leinster nobility joined Clare on his expedition to Meath, which demonstrated the extent of the earl's grip upon Leinster. Shortly after this campaign, Domnall Mór Ua Briain of Thomond and Ruaidrí Ua Conchobair invaded Osraige, checking Clare's success and forcing the townsmen of Kilkenny, with Domnall, to flee to Waterford. In summer 1174 they again sought to overthrow Clare's settlement of Leinster: in July, Ua Briain and the Ua Conchobair annihilated a large English army at Thurles, then, flushed with success, Ua Conchobair crossed the Shannon into Meath. In spite of this turmoil, Domnall remained loyal to Clare and together they survived the onslaught in Leinster. Domnall rendered further feudal service to Clare by engaging with the king of Thomond in Limerick in 1175, during which the O'Briens killed his son, and against the MacCarthys (Meic Carthaig) in alliance with the O'Briens a year later. The alliance proved mutually advantageous to the Mac Gilla Pátraic and the English, though an anecdote told by Gerald of Wales (qv) suggests that Domnall never wholly trusted Clare: at an assembly of the English and Irish at Waterford held before Robert le Poer (qv), a frog was discovered close to Waterford and was brought before the company; on seeing it, Domnall declared with great sadness that it was ‘a sure sign of the coming of the English, and the imminent collapse of his people’. In spite of his reservations about the English newcomers, this arch pragmatist maintained his relationship with them until his death some time in 1185. He was succeeded by his son Máel Sechlainn Mac Gilla Pátraic (qv) (d. 1194), who continued to hold North Osraige as a vassal of the heirs of Clare.