Mac Murchada, Domnall Cáemánach (d. 1175), king of the Irish of Leinster, was a member of the ruling family of Mac Murchada (MacMurrough), which belonged to the Síl Cormaic lineage of the Uí Chennselaig dynasty. Domnall himself was ancestor of the Cáemánach (Kavanagh) line, one of the few Irish families to bear an adjectival surname. He was the eldest son of the powerful Diarmait Mac Murchada (qv), king of Leinster; his mother's name is not recorded, which has prompted speculation that he was illegitimate. According to early modern sources, he gained his sobriquet because he was fostered in Cell Cháemáin (Kilcavan, Co. Wexford). Domnall had at least five half-brothers and half-sisters, his father having been involved in four marriages or unions. His siblings included Énna (blinded in 1168), Conchobar (executed 1170), Órlaith, who married Domnall Mór Ua Briain (qv), Derbfhorgaill, who became the wife of Domnall Mac Gilla-Mocholmóc, and Aífe (qv), who was given in marriage to Richard Fitzgilbert (Richard de Clare (qv)), generally known as Strongbow. In his turn, Domnall had at least two sons: a youth, not named in surviving records, who was executed in 1170, and Domnall Óc, who apparently flourished around the end of the twelfth century.
Domnall came to prominence from 1161 as a military commander in his father's service. In that year he attacked Wexford and expelled a dynast named Ua Domnaill, who belonged to a rival family of the lineage of Síl Cormaic. It seems that Domnall remained in Ireland when his father took flight in 1166. After his father's return, Domnall led the Uí Chennselaig contingent that met the Anglo-Norman forces under Maurice de Prendergast (qv) when they landed at Bannow Bay on 1 May 1169. That summer he campaigned with Prendergast against Domnall Mac Gilla Pátraic (qv), king of Osraige, who had blinded his brother Énna the previous year. The first expedition into Osraige came close to disaster when Mac Gilla Pátraic counter-attacked in the Slievemargy mountains; according to Anglo-Norman sources, most of Domnall Cáemánach's men deserted him, the situation being saved only through the courage of Prendergast. If Domnall was culpable for the failure of this first expedition, he retrieved his reputation to some degree in the autumn of 1169 with an attack on the petty kingdom of Uí Fháeláin (in north Co. Kildare) and a second incursion into Osraige. He was unable, nevertheless, to prevent the defection of Prendergast to the king of Osraige.
After the arrival of Strongbow, Domnall took part in the march on Dublin. The capture of the town in September 1170, however, had tragic consequences for Domnall when his youngest brother Conchobar and his own young son were executed by the high-king Ruaidrí Ua Conchobair (qv) in revenge. After the death of his father (early May 1171), Domnall was the only Leinster dynast of consequence who remained with Strongbow. He brought news to Dublin that the Irish of south Leinster had turned on the local Anglo-Norman contingent and besieged them at Carrick; he stayed in the town long enough to play a prominent part in breaking the siege by Ua Conchobair in late summer. It is probable that Domnall's support for Strongbow owed less to ideological commitment than to a recognition of the political realities. By late 1171 Strongbow had received a formal grant of Leinster from King Henry II (qv); besides, the Anglo-Normans were strongly entrenched and controlled the east-coast ports. Clearly, Domnall did not have the resources to dislodge them even if he felt so inclined. The only alternative for him was to support Ua Conchobair, as indeed his uncle Murchad Mac Murchadha (qv) (brother of Diarmait) had done. His father's protracted conflict with the high-king, however, and the latter's execution of two members of Domnall's immediate family, no doubt made any such alliance a distasteful option. In any event, he was among the Leinster sub-kings and other Irish rulers who submitted to King Henry during his visit to Ireland in the winter of 1171.
The claim of an Anglo-Norman chanson de geste that Strongbow ‘bailled the pleas of Leinster’ to Domnall, widely interpreted as meaning that he was created seneschal, seems to imply that English crown authority was established throughout Leinster, which was far from being the case. Local magnates, including the family of Domnall's uncle Murchad, firmly resisted the extension of English power, especially in the south and west of the province. It seems reasonable that Domnall, whose kingly status is acknowledged in Irish sources, was chosen by Strongbow as a prospective mesne-king for the Leinster Irish, including those dynasties which had not yet been brought under the direct control of the English crown.
In 1172 Domnall, with the backing of Strongbow, captured Ua Brain (O'Byrne) and his son, as local dynasts from northern Co. Wexford who had been disloyal to his father; he had them executed at Ferns. The killing of his uncle Murchad by the English later that year did not bring about an immediate change in his political alignment, although it seems that tensions were increased. It is probably significant that, although Domnall and his cousin Muirchertach (son of Murchad) took part in Strongbow's campaign in Meath in 1173, the chanson should refer to them as the earl's ‘enemies of Leinster’. His precise role in the events that followed is not certain, due to lack of clarity in the Irish annal-record; but war broke out that year between the Leinster Irish and the settlers; Strongbow suffered a signal defeat in Uí Chennselaig, with the loss of over 200 men, at the hands of Domnall or his son Domnall Óc. Perhaps he was prompted to revolt by successes gained against the Anglo-Normans in Osraige and Munster.
In 1175 Domnall Cáemánach, styled rí Laigen (king of Leinster) in Irish sources and aged probably in his mid forties, was slain; the identification of his killers in the Annals of Tigernach (Tigernach Ua Bráein (qv)) as Uí Nialláin (a minor line of Uí Fhailge), rather than Uí Núalláin (a Fothairt line) as the Annals of the Four Masters state, may indicate that his dynasty was active in west Leinster hostilities against the English. His son Domnall Óc, through his son in turn, also named Domnall, was the ancestor of the later medieval line of Mac Murchada Cáemánach, kings of the Irish of Leinster.