Donnell O'Brien may refer to
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Domnall Mór Ua Briain
Domnall Mór Ua Briain, or Domnall Mór mac Toirrdelbaig Uí Briain, was King of Thomond in Ireland from 1168 to 1194 and a claimant to the title King of Munster. He was also styled King of Limerick, a title belonging to the O'Brien dynas ...
(?–1194), king of Thomond, king of Munster, king of Limerick, 1168-1194
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Domhnall mac Conchobair Ó Briain (?–1579), claimed the title of Earl of Thomond in 1553
{{Hndis, Obrien, Donnell