Born Rosa O'Doherty, Rosa was the daughter of Sir John O'Doherty, and the younger sister of Sir Cahir O'Doherty. The O'Doherty's were the traditional rulers of Inishowen. Sir Cahir fought on the Crown's side during Tyrone's Rebellion (1594-1603). In 1608, angered at his treatment by local officials, he launched O'Doherty's Rebellion by burning Derry. Sir Cahir was defeated and killed at the Battle of Kilmacrennan, and Inishowen was confiscated from the family.