Brian Bóruma was the first man to establish himself as High King of Ireland by force of arms alone in many centuries. Previous men reckoned High King had belonged to the great Uí Néill kindred, that large group of families who traced their descent from Niall of the Nine Hostages, which dominated much of central and northern Ireland from the 7th century onwards. No king from the south, where Brian's kindred, the hitherto rather obscure Dál gCais of the region of Thomond, had come close to dominating Ireland since the time of Feidlimid mac Crimthainn in the early 9th century, and none had been included in the more widely accepted lists of High Kings in historic times.
Brian, building on his own resources, and those of the Viking towns of the south such as Limerick and Cork first took control of Munster, overthrowing the domination of the Eóganachta, a kindred which had dominated the kingship of Munster as effectively as the Uí Néill had dominated the High Kingship, and for just as long. With the Uí Néill disunited, and the resources of Munster, Brian first brought the Uí Néill High King Máel Sechnaill mac Domnaill to recognise him as an equal, and then as the master of Ireland. Brian met his death at the Battle of Clontarf on 23 April 1014, Good Friday, fighting against the King of Leinster and his allies. In myth and medieval pseudohistory this battle would become the last and greatest between the Irish and the Vikings and Brian the greatest of all Irish kings.
The Munster king lists have Brian followed by Dúngal Ua Donnchada of the Cashel branch of the Eóganachta rather than by one of his sons. Dúngal did not died until 1025, at about the time that Donnchad started to make his presence felt outside Munster. His half-brother Tadc was assassinated in 1023—the Annals of Tigernach add that this was done on Donnchad's order—while he had himself lost his right hand in what may have been a failed assassination attempt in 1019.
Donnchad arranged an alliance with Echmarcach mac Ragnaill, King of Man and the Isles, or at least of the Hebrides, enemy of his half-brother Sitric Silkbeard, King of Dublin. This was sealed by the marriage of his sister Caech to Echmarcach in 1032. Another ally was Echmarcach's cousin, Donnchad mac Gilla Patráic, King of Osraige and, from 1036, King of Leinster.
Donnchad's main rivals were Diarmait mac Maíl na mBó, King of Leinster from 1042, and Áed in Gaí Bernaig, King of Connacht from 1046. Diarmait in particular was a serious threat; allied with Niall mac Eochada, King of Ulster, he installed his son Murchad as ruler of Dublin in 1052, driving out Donnchad's brother-in-law and ally Echmarcach mac Ragnaill. Toirdelbach first joined with Áed in the early 1050s, raiding into Tuadmumu in 1052 and inflicting a heavy defeat on Donnchad's son Murchad in Corco Mruad, the north-west of modern County Clare in 1055. By 1058 Toirdelbach had gained Diarmait's support, for he was present when Diarmait, the Leinstermen and the Osraige drove Donnchad from Limerick, which he burned so that it would not fall into the hands of his enemies, and defeated him at Sliabh gCrot in the Galtee Mountains.
Geoffrey Keating's Foras Feasa ar Éirinn (Volume III, Chapter XXXIII) recounts that Donnchad granted the crown of Ireland to the Pope—Pope Urban II according to Keating, who places these events in 1092—and asked for papal aid to return him to power. This story is repeated in many 19th century and earlier works of popular history and is given as an explanation of how the English Pope Adrian IV came to issue the papal bull Laudabiliter granting rule of Ireland to King Henry II of England. Elsewhere (Volume III, Chapter XXVII) Keating is more skeptical regarding cother stories associated with Donnchad's time in Rome. He disbelieves claims that Donnchad took up with a daughter of a Holy Roman Emperor and had at least two sons from whom some later Old English families were descended. Keating writes that "this story cannot be true, for before setting out on that expedition [Donnchad] was a very old decrepit man of over eighty years of age, and it is not likely that an emperor's daughter would covet intercourse with such a veteran".