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The O'Brien Clan is a royal dynasty founded in the Middle Ages by a member of the Dalcassian tribe known as Brian Boru. After becoming King of Munster, through conquest Boru established himself as High King of Ireland, breaking the centuries hold of the title by the O'Neill. Boru's descendants thus carried the name O'Brien, continuing to rule the Kingdom of Munster until the 12th century where their territory had shrunk to the Kingdom of Thomond which they would hold for just under five centuries.

In total, four O'Brien monarchs ruled Munster and held the Kingship of Ireland between the years 978 and 1119. After the partition of Munster into Thomond and Desmond, the dynasty would go on to provide around thirty monarchs of Thomond until 1542. During part of this period in the late 13th century they had a rivalry, with the Norman de Clare house disputing the throne. The last O'Brien monarch to reign was Murrough O'Brien who surrendered his sovereignty to the House of Tudor's Kingdom of Ireland, becoming instead Earl of Thomond and maintaning a role in governance. Today the head carries the title of Baron Inchiquin.