In Southwest Ireland the Eoghnacht were pushed aside as kings of Munster in
the mid-tenth century by the Dál gCais of North Munster. This was aided by the attacks
of the Ui Neill on the Eoghanact and by the dynastic wars which were soon
to keep the Southern and Northern Ui Neill busy. On his death notice in 951 Cennetig, king of
the Dál gCais, is called king of North Munster. His son Brian Boruma, or
Brian Boru (ancestor of the O'Briens, et al), who succeeded in 976, was to make
himself an overking of most of Ireland and a rival of the powerful Ui Neill dynasties.
Domnall ua Neill, king of the Northern Ui Neill, was overking of the Ui Neill dynasties from
956-980, and held the title of 'Ard Ri' (high-king of Ireland). He reigned at the time when Brian Boru
began his rise to power in Ireland. About 978 Brian had a major success against the Ostmen
(Vikings) of Limerick and their Eoghanacht allies. It was the Limerick king, Imar, and his sons, who had a couple of years
before, assassinated Brian's brother. Through aggressive raids over the next three years Brian
had the province of Munster in his hands. Brian came into conflict with the Ui Neill when he
took his armies East and tried to conquer Ossory (Osraighe) about the year 980.
This was at the time that Domnall ua Neill was succeeded by Mael Sechnaill
II (of the southern Ui Neill) as 'Ard Ri'. However, by 984, Brian was in control of the south of Ireland.
Allied with the Ostmen of Waterford, Brian later pushed his armies into the provinces of Connacht,
Leinster and Meath (Midhe), gradually extending his power base further. Mael Sechnaill's attempts to contain Brian were
not successful, and in 997/998 Brian and Mael Sechnaill met at Clonfert and divided Ireland
Late in 999 Brian brought the Ostmen king of Dublin, Sitric Silkenbeard, into
submission. By 1002 Mael Sechnaill recognized Brian as high king of Ireland by right of
conquest. By 1011 Brian dominated most of Ireland. He had shattered the
Ui Neill monopoly and had made the kingship of Ireland a prize to be
fought for. About 1012, relations between Brian and Leinster became
strained and he was killed at the battle of Clontarf on Good Friday in the year 1014.
Although he won the battle, Brian was stopped short just when he seemed
about to make the kingship of Ireland a reality, and the possession of
his dynasty (Dalcassian). The death of Brian and the subsequent weakness of
his Dalcassian successors allowed Mael Sechnaill II to be titled 'high-king' of Ireland until his
death in 1022. The Annals appear to go slient on the term 'Ard Ri' for the next 66 years, likely
because it was contested among provincial kings. Among the contenders during this time was,
Donchadh, son of Brian Bóruma (Dal gCais of Munster); Diarmait, son of Donchadh
Máel na mBó (Ui Cheinnselaig of Leinster); and Toirrdelbach, grandson of Brian Bóruma.
The high kingship never again saw the same power that Brian Boru held. Beginning in 1087 the
last six 'Ard Ri' of Ireland appear to have alternated between provincial candidates, with periods
where there are no clear high king; these included kings from Aileach (Cenel Eoghain),
next from Munster (Dal gCais), then Connacht (Ui Briuin), then Tír Conaill (Cenel Eoghain), and
ending with Ruaidrí O'Connor of Connacht (Ui Briuin) in 1186.