Diarmait mac Maíl na mBó Diarmait mac Maíl na mBó ( died 7 February 1072 ) was King of Leinster , as well as High King of Ireland ( with opposition ) . He was one of the most important and significant kings in Ireland in the pre - Norman era . His influence extended beyond the island of Ireland into the Hebrides , the Isle of Man , Wales , and even into England . Background . Diarmait belonged to the Uí Cheinnselaig , a kin group of south - east Leinster centred on Ferns . His father , Donnchad mac Diarmata , more commonly known by the epithet Máel na mBó , whence Diarmait ' s patronym came . The last of Diarmait ' s ancestors to have been counter as king of all Leinster was Crimthann mac Énnai , whose death is placed in the late 5th century , but his ancestors , most recently his great - grandfather Domnall mac Cellaig ( died 974 ) , had been counted among the kings of the Uí Cheinnselaig . Diarmait ' s mother was Aife , daughter of Gilla Pátraic mac Donnchada , king of Osraige . He had at least one sibling , a brother named Domnall whose son Donnchad mac Domnaill Remair was later king of Leinster . The Uí Cheinnselaig had been prominent in earlier times , but their power had been broken at the battle of " Áth Senaig " in 738 . The rival Uí Dúnlainge , based in northern Leinster around Naas and Kildare , who also enjoyed the support of the powerful Clann Cholmáin kings of Mide , dominated Leinster until the time of Brian Bóruma . The decline of Clann Cholmáin , and the defeat inflicted on the Uí Dúnlainge , led by Máel Mórda mac Murchada , at the battle of Clontarf in 1014 , changed the political landscape to favour the Uí Cheinnselaig once more . The return of the Vikings to Ireland in the early 10th century brought with it the creation of new towns on the coasts . The towns , centres of trade and manufacture , would give significant political power to those who could control their wealth . Kings of Leinster were in a particularly advantageous position to exploit this new wealth as three of the five principal towns lay in or near Leinster . In Leinster proper , in the south - eastern corner dominated by the Uí Cheinnselaig , lay Wexford . To the west of this , in the smaller kingdom of Osraige , which had been attached to Leinster since the late 10th century , was Waterford . Finally , the most important Viking town in Ireland was Dublin , which lay at the north - eastern edge of Leinster . Compared to this , kings in the north and west of Ireland had easy access to no towns , while those in the south , in Munster , had access to two , Cork on the south coast and Limerick on the west coast . Niall mac Eochada . He made an alliance with Niall mac Eochada , king of Ulaid , which helped to put pressure both from the north and south on the kingdoms of Mide , Brega and Dublin — ruled by the High King . This alliance paid off handsomly as the High King , who up till then had Dublin as a subject kingdom , was unable to prevent it slipping into the hands of Diarmait . The Man Who Would Be High King . In 1042 he was able to claim the title " King of Leinster " and install his son , Murchad , as King of Dublin . Thus ruler of two of the most powerful and wealthy towns on the island , he was able to make a bid for the High - Kingship . It was during a battle against the king of Mide , Conchobar Ua Maelsechalinn , that he was killed , near to Navan . County Meath , on 7 February 1072 . The Sons of Harold . The surviving sons of King Harold Godwinson of England escaped to Leinster after the Battle of Hastings in 1066 where they were hosted by Diarmait . In 1068 and 1069 Diarmait lent them the fleet of Dublin for their attempted invasions of England . Death of Murchad and Diarmait . He is also famous as the ancestor of Diarmait MacMurrough . Diarmait ' s Death in the Annals of the Four Masters . " " Diarmaid , son of Mael - na - mbo , King of Leinster , of the foreigners of Ath - cliath , and of Leath - Mogha - Nuadhat , was slain and beheaded in the battle of Odhbha , on Tuesday , the seventh of the Ides of February , the battle having been gained over him by Conchobhar O ' Maeleachlainn , King of Meath . There were also slain many hundreds of the foreigners and Leinstermen , along with Diarmaid , in that battle . In it was killed Gillaphadraig O ' Fearghaile , lord of the Fortuatha , & c . Of the death of Diarmaid was said : " "