Mathghamhain mac Cian

{{Short description|Irish nobleman (died 1014)}}

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Mathghamhain mac Cian was a nobleman of the Eóganacht Raithlind and a grandson of Brian Boru, one of the High Kings of Ireland. He was the son of Cian mac Máelmuaid and his wife Sadhbh, daughter of Brian Boru. The Leabhar Oiris records Mathghamhain fighting at the Battle of Clontarf for the High King and committing a familial revenge killing.{{Cite web |title=The Leabhar Oiris - An electronic edition |url=http://research.ucc.ie/celt/document/G100029 |access-date=2025-06-12 |website=University College Cork |language=en}}{{secondary source needed|date=June 2025}} He died at the Battle of Clontarf, in 1014.{{cn|date=June 2025}}

Mathghamhain's descendants eventually became sovereigns over territory in west Cork, notably around the Iveragh Peninsula and the town of Bandon. Today, his descendants use the surname O'Mahony, O'Mahoney, Mahoney, Mahony, Maughon or Mahaney, Irish Ó Mathghamhna or modernised Ó Mathúna.