In a bid to secure the High Kingship of Ireland for the clan, Muirchertach encouraged ecclesiastical reform in 1111 with the creation of territorial dioceses over the entire island.
They had support for their bid from several foreign connections including the Norwegian king Magnus Bareleg and the Anglo-Norman baron Arnulf de Montgomery, who were both united to the clan through marriage in 1102.
[1] Their claim to the High Kingship was countered by the O'Neill (Uí Néill) clan in Ulster under the leadership of Domnall MacLochlainn of Ailech.
[1] MacLochlainn's plans to restore the High Kingship to the north was thwarted by his ally Tairrdelbach Ua Conchobair of Connaght who formed an alliance with the O'Brien's.
It was a disastrous defeat for the Ó Briain army with a loss of over 3,000 men and Toirdelbhach was forced to flee Munster.