house of O'Callaghan descends in unbroken male line from Morrough,
second son of Ceallachan Caisil (Callaghan Cashel), King of Munster who
died in the year 952. Morrough was thus the younger brother of Donnchad
II of Munster, ancestor of the House of MacCarthy Mór. Closely related
or not there was, at first, bitter and prolonged rivalry between the
descendants of Morrough and King Donnchad. Because the succession to the
throne of Munster was not determined by primogeniture but by tanistry,
it was natural for the O'Callaghans to aspire to the Crown of their
ancestor, Ceallachan, from whom they derived their surname.
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