O'Donnell (Ó Domhnall), Ruaidhrí (1575–1608), 1st earl of Tyrconnell , was second son of Sir Aodh O'Donnell (qv), lord of Tír Conaill, and his wife Fiona MacDonnell (qv), daughter of James MacDonnell, lord of the Western Isles of Scotland. Little is known of his childhood except that his father promised him as a pledge for good behaviour in July 1588. After his brother Red Hugh (Aodh Ruadh) (qv) was inaugurated as lord of Tír Conaill in 1592, Ruaidhrí became tanaiste. He stayed very much in his elder brother's shadow throughout this period, and only occasional reference is made to him. In summer 1597 he with his brother successfully restored Theobald mac Walter Burke to his lordship of Mayo by driving out his cousinly rival, Sir Theobald na Long Bourke (qv). Ruaidhrí was instructed to defend Theobald, but Sir Theobald returned with English troops and forced them to take refuge in Tír Conaill. Despite this reverse, Ruaidhrí demonstrated considerable soldierly skill in the difficult retreat northwards under constant attack. While the detail is lacking, it seems Sir Conyers Clifford (qv), governor of Connacht, engaged Ruaidhrí in a plot against his brother in 1598. When news of this reached Red Hugh, he put his errant brother in fetters for an unspecified time.
By 1600, however, good relations had resumed between the brothers. On 9 October 1600 they faced the rebellion of their cousin and brother-in-law, Niall Garvach O'Donnell (qv), who seized Lifford from their garrison and (with English forces) defended it against the best attempts of the brothers to retake it. The struggle culminated in a battle outside Lifford (24 October 1600), during which Niall mortally wounded Ruaidhrí's brother Manus and engaged in single combat with Ruaidhrí, from which the latter was lucky to escape alive. Red Hugh then marched to Mayo (December 1600), leaving Ruaidhrí as governor of the lordship. His tenure was not distinguished, as the rebellion of Niall gathered momentum and spread into the lands of the MacSweeneys. Then Niall and Sir Henry Docwra (qv) seized Donegal abbey and forced Ruaidhrí to lift the siege of a castle in Sheephaven Bay (April 1601). When the Spanish forces landed at Kinsale (September 1601), Ruaidhrí accompanied his brother and Hugh O'Neill (qv), 2nd earl of Tyrone, on the long winter march southwards. After the defeat of their forces on 24 December 1601, it was decided that Red Hugh would go to Spain and Ruaidhrí would accompany Tyrone back to Ulster. In the face of considerable odds, Ruaidhrí, with Brian Óg O'Rourke (qv), tried to recover the military initiative in Connacht. Defeats such as the loss of the key town of Ballyshannon to Niall (spring 1602), combined with their failure to halt the march of Sir Oliver Lambart (qv) on Sligo in June 1602, signalled the end was near. To make matters worse, Ruaidhrí quarrelled with O'Rourke. On 10 September 1602 he became lord of Tír Conaill on Hugh Roe's death in Spain, and doggedly stayed in the field until his submission at Athlone (December 1602).
When offered the chance to revenge himself on O'Rourke, Ruaidhrí eagerly grasped it, delivering the decisive blow in a three-pronged government attack. Opportunistically, Ruaidhrí took advantage of his new status to attack Niall (April 1603). Two months later he accompanied Tyrone to England, and acknowledged James I as his king at Hampton Court (7 June 1603). On his return, he was knighted (29 September 1603) in Christ Church cathedral, Dublin, and was created 1st earl of Tyrconnell. In April 1604 Ruaidhrí was granted most of Tír Conaill but was forced to renounce his claims to the Inis Eóghain lordship of Sir Cahir O'Doherty (qv). But the new earl had greater difficulty in accepting the loss of his claims to lands of his bitter rival Niall. This resulted in violence between them which Sir Arthur Chichester (qv), lord deputy of Ireland, was unable to resolve in 1605.
Tyrconnell was a brave man and a soldier of determination and ability. However, because of his part in the nine years war he was viewed with suspicion by the English government. Discontent with the settlement of Tír Conaill and his inability to make the transition from warlord to nobleman led to financial difficulties. The combination of these problems made him bitter and encouraged his plotting. About Christmas 1606 a heavily indebted Tyrconnell married Bridget, daughter of Henry Fitzgerald (qv), 12th earl of Kildare. On one of his trips to Maynooth in 1607, he revealed his discontent with the settlement of Tír Conaill to Richard Nugent (qv), Lord Delvin, later 1st earl of Westmeath. During this conversation, he spoke of a plot, involving Tyrone and the Spanish, against the English government. By September 1607 Tyrconnell realised that his treacherous talk was known to the government, and convinced Tyrone to flee with him to Spain. He fled that month from Rathmullen on Lough Swilly, leaving his wife behind, successfully avoided a seaborne attempt to arrest him and Tyrone, and landed in France three weeks later after a rough voyage. From France they travelled to Brussels, where they were feted as defenders of catholicism by the Spanish (November). From Louvain they penned their grievances and sent them to London (December). When they entered Rome, Tyrconnell and Tyrone were welcomed by a guard of cardinals on 29 April 1608, and were given an audience by the pope the next day. Even greater honour was bestowed on them when they helped to carry a canopy over the pontiff's head during the festival of Corpus Christi. Amid the adulation, Tyrconnell missed his wife and unsuccessfully tried to communicate with her in Ireland. Sometime in early June he fell ill with a fever, and despite a trip to the waters of Ostia, it grew worse. On 28 July 1608 he died alone, aged 33, and was buried in the church of S. Pietro on the Janiculum Hill, Rome.