O'Donnell (Ó Domhnaill), Niall Garbh (1380–1439), king of Tír Conaill, was eldest son of Toirdhealbhach O'Donnell (qv), king of Tír Conaill, and Gráinne, daughter of Niall Mór O'Neill (qv), king of Tír Eóghain. The first mention of Niall Garbh was during an O'Donnell campaign (1398) against the MacSweeneys of Fanad. In 1418 he was the leader of the O'Donnell forces that defeated Domhnall O'Neill (qv) of Tír Eóghain. However, Brian O'Connor (qv) of Sligo surprisingly routed Niall Garbh during 1420 in a night attack at Assaroe Bay, forcing him to swim for his life to a trading ship. Nevertheless, he succeeded his father in 1422. His first act as king was to plunder Meath and Louth with Domhnall O'Neill of Tír Eóghain. The alliance with O'Neill was brief, as Niall Garbh helped his cousin Eóghan O'Neill (qv) to expel Domhnall from Tír Eóghain. This impressive regional power was confirmed by Niall Garbh's circuit through Ulster (1422), taking the submissions of Maguire, MacMahon, Magennis, and O'Kane. On this campaign he also defeated the Anglo-Irish Bissets of the Glens of Antrim and attacked the Clandeboye O'Neills of east Ulster. After this he went to Connacht to collect the hostages due to him. Niall Garbh's renown and importance was further confirmed by his marriage to Fionnuala, daughter of An Calbhach O'Connor Faly (qv), lord of Offaly, and Mairghréag O'Carroll (qv). In the years that followed he adopted a shifting policy towards the O'Neills, depending on the strength of the rival claimants. In 1423 he renewed his alliance with Domhnall O'Neill, forcing tributes from the English of Meath and Louth. During 1424 he and Eóghan opposed the Ulster campaign of James Butler (qv), 4th earl of Ormond. But Niall Garbh's encouragement of the Clandeboye O'Neills against Eóghan O'Neill in 1427 signalled the end of their alliance. His victory that year over the MacQuillans of east Ulster further affirmed his provincial dominance.
The rise of Eóghan, however, presented him with a serious challenge. In 1431 a peace conference between the pair failed, leading to a state of continual war. Two years later Eóghan demonstrated the change in O'Neill fortunes. While Niall Garbh was helping the MacQuillans, Eóghan hired a mercenary fleet of MacDonnell gallowglass to fight them. In this campaign Niall Garbh made the mistake of leaving the MacQuillans to face the MacDonnells alone; the MacQuillan forces were destroyed by the O'Neills and the MacDonnells. After this success, Eóghan surprised Niall Garbh by marching straight to Tír Conaill. He also ordered the MacDonnells to sail there, catching the numerically inferior O'Donnell forces off guard. With Niall Garbh and his army somewhere in east Ulster, Eóghan had cornered the defenders of Tír Conaill in a huge pincer movement. As a result Fionnuala, Niall Garbh's wife, and Neachtan (qv), his brother, quickly sued for peace without his permission. Although outmanoeuvred, Niall Garbh took his army to Meath to confer with the English before returning to Tír Conaill through Connacht. On his way home he made his peace with Eóghan at Cáoluisce. Because of this fiasco his relations with Neachtan soured, resulting in civil war in Tír Conaill during 1434. But Niall Garbh's new alliance with the O'Neills was to prove disastrous. In December 1434 he and Eóghan campaigned into Meath and Louth in search of tribute. There he and his son, Toirdhealbhach, became separated from the main army. They were spotted by the English, who surrounded them. Toirdhealbhach was killed but Niall Garbh was captured, taken to Dublin, and from there conveyed to London (1435). During his imprisonment, Neachtan ruled Tír Conaill, but he was slow to open negotiations for his brother's release. In 1437 they settled their differences, resulting in O'Donnell pressure to free Niall Garbh. Indeed, this pressure delivered dividends as Niall Garbh was transferred from London to the Isle of Man in 1439. Neachtan seemingly agreed to pay the ransom, but Niall Garbh died there before it arrived.