Murchad (d. 1070), son of Diarmait son of Donnchad Máel na mBó (qv) and king of Dublin, belonged to the south Leinster dynasty of Uí Chennselaig. His father Diarmait (qv), arguably the most powerful ruler the dynasty produced, was an overking of Leinster whose lordship extended over Munster and Dublin and into the Irish Sea area. His mother was Derbfhorgaill (d. 1080), daughter of Donnchad (qv) son of Brian Bórama (qv). He had a brother, or half-brother, named Énnae (d. 1092), who later reigned as king of Uí Chennselaig. Murchad was married to Sadb, daughter of one Mac-Bricc, at least for a time. She bore him a son, Donnchad, but it is not stated that she was the mother of his other sons, Domnall and Énna, or of his daughter, Gormlaith.
Murchad sprang to prominence when he was appointed ruler of Dublin by his father, who in 1052 had expelled the last independent king of Dublin, Echmarcach (qv) son of Ragnall. His appointment to the kingship of Dublin probably took place in or after 1054, as Ímar son of Harald, the Hiberno-Scandinavian dynast whom his father had entrusted with the Dublin realms, died that year. The record shows that, increasingly from the later 1050s, Murchad was active on behalf of his father, especially within the kingdoms of Dublin and north Leinster. Striving to secure his dynasty's claims in the Irish Sea area, he led an expedition to the Isle of Man in 1061, where he defeated the son of Ragnall, probably to be identified with the former Dublin king. On the same occasion, Murchad levied a tribute on the Islesmen. Prior to this, he had enforced Uí Chennselaig lordship over the Uí Dúnlainge of north Leinster. In 1059 he slew two prominent members of the latter dynasty, who belonged to the lineages of Uí Muiredaig and Uí Fháeláin. It appears that Murchad's exactions upon north Leinster were at least partly responsible for the exodus in the 1060s of dynasts from that region to bolster the (failing) resistance of Donnchad son of Brian to Uí Chennselaig expansion.
It appears that Murchad had few involvements beyond the ambit of Dublin and north Leinster. In attempting to assert his authority in the neighbouring province of Meath, he met with what could only be described as limited success. In 1059 he slew a dynast of the Southern Uí Néill. Then (1069) he led a major incursion into Meath and the adjoining areas, burning the ecclesiastical settlements of Granard, Ardbraccan, and Fobar (Fore, Co. Westmeath). The fact that Murchad's forces were afterwards defeated and driven back to Dublin was attributed by a Clonmacnoise annalist to the vengeance of St Féchín (qv) of Fore.
The following year Murchad died prematurely, aged probably in his mid forties, and was buried in Dublin. He was succeeded as king of Dublin by his son Domnall, who died in 1075, presumably at quite a young age. Murchad's daughter Gormlaith became abbess of Kildare and died in 1112. However, his son Donnchad, recognised as overking of Leinster from 1098, lived till 1115; he was ancestor of the family line of Mac Murchada (MacMurrough) which provided all the subsequent kings of Leinster.