O'Donnell (Ó Domhnaill), Neachtan (1392–1452), king of Tír Conaill, was second son of Toirdhealbhach O'Donnell (qv), king of Tír Conaill; his mother was probably Gráinne, daughter of Niall Mór O'Neill (qv) (d. 1397). The first mention of Neachtan comes in 1420 when he and his brother, Niall Garbh O'Donnell (qv), were engaged in a border war with Brian O'Connor (qv) of Sligo. Broadly speaking, Neachtan enjoyed good relations with Niall Garbh and respected his succession to the kingship on the death of their father (1422). Indeed, Niall Garbh was to ransom Neachtan when he was captured by the English in 1426. However, these good relations were spoiled by the spectacular campaign of Eóghan O'Neill (qv) of Tír Eóghain against the O'Donnells in 1433, when he invaded Tír Conaill while Niall Garbh was fighting in east Ulster. This sudden move forced Neachtan and Fionnuala, Niall Garbh's wife, to make peace with the O'Neills. Their capitulation was not sanctioned by Niall Garbh and proved embarrassing. As a result the brothers became alienated from each other and civil war broke out in 1434. Their feud took an unexpected turn that year when Niall Garbh was captured by the English. Neachtan ruled Tír Conaill during his captivity and may have been reluctant to press for his release. However, he may also have been initially distracted by the conquering intentions of Eoghan and his son, Énrí O'Neill (qv), in 1435. Two years later the brothers became reconciled, beginning earnest negotiations for Niall Garbh's release. By 1439 Neachtan seemingly had agreed to pay Niall Garbh's ransom, as the prisoner was transferred from London to the Isle of Man. This came too late, as Niall Garbh died suddenly that year, allowing Neachtan to become king of Tír Conaill.
Neachtan's reign was characterised by his bitter rivalry with Énrí O'Neill of Tír Eóghain. In 1452 Énrí dispatched Niall Garbh's sons to Tír Conaill, where they assassinated Neachtan. Rubbing salt into the O'Donnell wounds, Énrí then briefly took Neachtan's wife, the daughter of MacWilliam Burke, as his concubine. Neachtan's death was not the end of the succession dispute, as the kingship was fought over by his sons and those of Niall Garbh. The principal leader of Neachtan's sons, Ruaidhrí O'Donnell (Ó Domhnaill) (c.1420–p.1454), succeeded his father, fighting a bitter war with Domhnall O'Donnell (qv), son of Niall Garbh. However, he was killed by Domhnall at his siege of the castle of Inis (1454). Domhnall then assumed the kingship, but Ruaidhrí’s death was avenged (18 May 1456) by his brother, Toirdhealbhach Cairbreach O'Donnell (Ó Domhnaill) (c.1425–p.1461), who then became king of Tír Conaill and attempted to ensure his position by allying with Énrí O'Neill. However, Énrí tired of this alliance and opted for change in 1460, releasing Niall Garbh's son Aodh Ruadh O'Donnell (qv) (d. 1505) from captivity. At the battle of Ceann Maghair (1461), Aodh Ruadh routed and captured Toirdhealbhach Cairbreach, and afterwards made his captive unfit for the kingship by removing his hand and foot. After this nothing further is known of Toirdhealbhach Cairbreach, but the struggle between the sons of Niall Garbh and Neachtan continued for decades.