O'Mahony, the most common contemporary form of the name, comes from the Irish O Mathghamhna, stemming, like MacMahon, from mathghamhan, meaning "bear". The surname was adopted in the eleventh century by one of the dominant families of the Munster Eoghancht peoples, the Cenel Aeda, whose name is preserved in the barony of Kinalea; the individual from whom the name derives was the child of a marriage between Cian, chief of the Cenel Aeda, and Sadhbh, daughter of Brian Boru. Mathghamhan was a name more commonly associated with the Dal gCais, the sept or tribe of Brian Boru. With the rise of the MacCarthys in the twelfth century, the influence of the O'Mahonys declined, and was largely confined to the area of west Cork with which they are still most strongly associated, the Iveragh peninsula. In this area, perhaps because of its remoteness, they retained a large measure of power and wealth until the final collapse of Gaelic power in the wars of the seventeenth century. According to the account of Sir Richard Cox, writing in that century, there were at least twelve O?Mahony castles in the area. Their ruins can still be seen at Dunbecon, Dunmanus, Three Castle Island and Leamcon, amongst others. In addition, minor branches of the family - minor purely in terms of seniority - were created in Muskerry and Kinalmeaky baronies in Co. Cork, in particular the area around the modern town of Bandon.
Count Daniel O?Mahony (d.1714) is the best known of the family to fight in the European armies of the 18th century, the "hero of Cremona".
Colonel John O?Mahony (1815-77) , scholar, soldier and revolutionary, was one of the founders of the Fenian Brotherhood in the United States, leader of the 99th Regiment of the New York National Guard in the U.S. civil war and a translator of some distinction.
Francis Sylvester Mahony (1804-66), better known under his pen-name of ""Father Prout", was the composer of one of the most famous ballads celebrating Cork city, The Bells of Shandon.
The most famous modern bearer of the name was Eoin ("The Pope") O'Mahony (1904-1970), barrister, lecturer, writer and genealogist, who preserved and interpreted with accuracy and enthusiasm the traditions of his own and many other families, founding and organizing the annual clan gathering of the O'Mahonys.