Fragmented Kingship

In the wake of Máel Sechnaill's death in 1022, Donnchad mac Brian attempted to claim the title of 'King of Ireland'. However, his efforts were in vain as he failed to gain widespread recognition. During this tumultuous period, the notion of a singular high king of Ireland remained elusive, as evidenced by the glossing of Baile In Scáil, which listed Flaitbertach Ua Néill as high king, despite his inability to control even the northern regions.

From 1022 to 1072, no one could convincingly claim kingship over all of Ireland, marking this era as a significant interregnum, recognized as such by contemporary observers. Flann Mainistrech, in his regnal poem Ríg Themra tóebaige iar tain written between 1014 and 1022, listed Christian kings of Tara but did not identify a high king in 1056. Instead, he mentioned several regional kings: Conchobar Ua Maíl Schechnaill of Mide, Áed Ua Conchobair of Connacht, Garbíth Ua Cathassaig of Brega, Diarmait mac Maíl na mBó of Leinster, Donnchad mac Briain of Munster, Niall mac Máel Sechnaill of Ailech, and Niall mac Eochada of Ulaid.

The internal strife within the Cenél nEógain allowed Niall mac Eochada of Ulaid to expand his influence. Niall formed an alliance with Diarmait mac Maíl na mBó, who controlled much of the east coast of Ireland. This alliance enabled Diarmait to seize direct control of Dublin in 1052, a significant departure from past leaders like Máel Sechnaill and Brian, who merely looted the city. Diarmait took on the unprecedented role of the kingship "of the foreigners" (ríge Gall), marking a notable shift in Irish power dynamics.

Following Diarmait mac Maíl na mBó's control over Dublin, his son, Murchad, maintained influence in the east. However, after Murchad's death in 1070, the political landscape shifted again. The High Kingship remained contested, with various rulers holding and losing power quickly.

One prominent figure of this period was Muirchertach Ua Briain, grandson of Brian Boru. Muirchertach aimed to consolidate power and revive his grandfather's legacy. His reign (1086–1119) involved efforts to dominate the High Kingship, though his authority faced constant challenges. He formed alliances, notably with the Norse-Gaelic rulers of Dublin, and engaged in conflicts to strengthen his position.

The early 12th century saw significant ecclesiastical reforms, with the Synod of Ráth Breasail in 1111 and the Synod of Kells in 1152 restructuring the Irish church. These reforms aimed to align the Irish church more closely with Roman practices, enhancing ecclesiastical organization and political influence.

In the mid-12th century, Toirdelbach Ua Conchobair (Turlough O'Connor) of Connacht emerged as a powerful contender for the High Kingship. He launched numerous campaigns to assert control over other regions and invested in fortifications, contributing to the political turbulence of the era.

A pivotal figure leading to the Anglo-Norman invasion was Diarmait Mac Murchada (Dermot MacMurrough), King of Leinster. In 1166, Diarmait was deposed by a coalition of Irish kings led by Ruaidrí Ua Conchobair (Rory O'Connor), the reigning High King. Seeking to reclaim his throne, Diarmait fled to England and sought assistance from King Henry II.