| O'Mahony | |
|---|---|
 O'Mahony arms | |
| Parent house | Eóganachta |
| Country | Kingdom of Desmond |
| Founder | Mathgamain mac Cian |
| Final ruler | The 20th Duke O'Mahony (circa 1740-1790) |
| Titles |
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| Dissolution | 1740-1790 |
O'Mahony (Old Irish: Ó Mathghamhna; Modern Irish: Ó Mathúna) is the original name of the clan, with breakaway clans also spelled O'Mahoney, or simply Mahony, Mahaney and Mahoney, without the prefix. Brodceann O'Mahony was the eldest of the four sons of Mathghamain, known as "The Four Descendants".
The O'Mahonys were Cenél nÁeda princes of the ancient Eóganacht Raithlind. They were also for a period kings of Munster and Desmond, and take their name from Mathghamhain, son of Cian, son of Máel Muad mac Brain, King of Munster from around 960 to 970, and then again from 976 to 978. From 970 to 976 he was king of Desmond. His son Cian became a close ally of Brian Bóruma and married his daughter Sadb. From this marriage descend the O'Mahonys. Their Dukedom of O'Mahony came to an end in 1740.