Sir DonalO'Donnell was the eldest son of Sir Hugh O'Donnell, the ruler of Tyrconnell. Sir Donal was the leading contender in the O'Donnell succession dispute of the 1580s which took place while his father was still alive. His personal jurisdiction covered "that part of Tirconnell from the mountain westwards, i.e. from Barnesmore to the river Drowes, and also all the inhabitants of Boylagh and Tir Boghaine ". His father's primary domain concentrated on Kilmacrenan and Mongalvin, and his cousin Niall Garbh O'Donnell held sway in Lifford and eastwards of there. The Four Masters described him as "a mighty champion and great in battle, and it was never heard that at any time he had turned his back on his enemies" and O'Donovan says he was known "to lead his father's forces". Amongst the other leading contenders were his younger half-brother Hugh Roe O'Donnell and his great-uncle Hugh Dubh O'Donnell. Donal received the allegiance of O'Boyle and MacSweeney Banagh, as well as of the English Crown, who felt he had a stronger claim to the Lordship, by primogeniture. The Dublin government felt that Donal could command more support locally than his brother as Donal's mother was an Ulster woman while Hugh Roe was the son of Iníon Dubh, a Scottish woman. Shortly after the Armada shipwreck of 1588, Sir Donal O'Donnell was knighted and appointed as Sheriff of Donegal by the Lord DeputyWilliam FitzWilliam. Fitzwilliam also had Sir Eoin O'Gallagher, an influential supporter of Hugh Roe arrested and imprisoned. In 1589 a force of Irish Army troops were sent into the area under Captain John Connill who assisted Sir Donal against his rivals. Sir Donal grewstronger in the late 1580s, and took control over much of western Tyrconnell. He was also powerful enough to interfere in a succession dispute in neighboring Fermanagh, where he established Hugh Maguire as chief ahead of his rival Connor Roe Maguire. Donal also drew strength from the support of Sir Turlough O'Neill, whose daughter he married.. In a Spanish genealogy of the O'Donnell Dukes of Tetuan, Sir Donal O'Donnell features as a collateral ancestor with the annotation "en 1589 su padre le concedio el titulo de marques - en 1587 se adhirio al pacto de su padre para librar a su hermano Hugh". Faced with the eclipse of her son Hugh Roe's position, Iníon Dubh acted usurpively. She had already burnt her husband's Castle, with whom she waged war. She hired large numbers of Redshank mercenaries from her native Scotland to confront her son's rival. Sir Donal was defeated and killed at the Battle of Doire Leathan on 14 September 1590. After escaping from Dublin castle, two years later Hugh Roe successfully had himself made The O'Donnell.Shortly after the Armada shipwreck of 1588, Sir Donal O'Donnell was knighted and appointed as Sheriff of Donegal by the Lord Deputy William FitzWilliam. Sir Donal O'Donnell was survived by his only son, Donal Oge O'Donnell.
