O'Donnell (Ó Domhnaill), Sir Aodh mac Maghnusa (c.1520–1600), lord of Tír Conaill, was son of Manus O'Donnell (qv), lord of Tír Conaill, and Siobhan, daughter of Conn Ó Néill. He may have been among the sons of Manus defeated in 1531. During 1542 the annals mention Aodh campaigning for his father against the lords of north Connacht. In 1543 the O'Gallaghers seized the castle of Lifford on the borders of Tír Conaill and Tír Eóghain. Aodh, probably encouraged by Con Bacach O'Neill (qv), joined them but in the following year Lifford surrendered. For the remainder of the 1540s Aodh remained estranged from his father and his half-brother, An Calbhach O'Donnell (qv) (d. 1566).
When An Calbhach overthrew Manus (1555), Aodh made his bid for power. In 1557 Shane O'Neill (qv), lord of Tír Eoghain, was routed by An Calbhach while attempting to aid Aodh's rebellion. However, Aodh's luck changed in May 1561, when O'Neill captured An Calbhach, imprisoned him in Tír Eóghain, and then set up Aodh as ruler of Tír Conaill. Once installed, Aodh proved an active ruler and defeated Cathal Ó Conchobhair at Sligo (1561). In September 1565 An Calbhach was restored by the English; Aodh fled to O'Neill but returned with his ally to devastate Tír Conaill. On An Calbhach's death (October 1566), Aodh succeeded him and raided Tír Eóghain. In response, O'Neill invaded Tír Conaill, but Aodh pinned his soldiers against the rising evening tide of Lough Swilly, drowning most of them.
Despite this victory, Aodh could not restore O'Donnell fortunes. Throughout his reign he had to contend with the ambitions of An Calbhach's descendants, and he was pushed out of Connacht by the English in the 1570s. With O'Rourke, he invaded Connacht, achieving a transitory success (1580), but his regional weakness was evident when he asked for English aid to counter the advances of Turlough Luineach O'Neill (qv) in 1581. A humiliating defeat followed at O'Neill's hands, signalling the nadir of O'Donnell fortunes. He, however, turned the tables on Turlough Luineach and routed him in June 1583. This victory did not stem the decline of Tír Conaill as the northern lordships were lost to An Calbhach's descendants by the middle of the 1580s. Complicating matters further, an aged Aodh suffered from poor health. The establishment of the presidency and council of Connacht (1585) curtailed any resurgence of the O'Donnell threat to the province, and Aodh's son, Red Hugh (Aodh Ruadh) O'Donnell (qv), was kidnapped and brought to Dublin in 1587. Aodh formed an alliance with his son-in-law, Hugh O'Neill (qv), 2nd earl of Tyrone, against Turlough Luineach, who however defeated their union in 1588. Increasingly during these years, Aodh's wife Fiona (qv), ‘Ingíon Dubh’, ruled in her husband's name. About 1590 Domhnall, Aodh's illegitimate son, attempted to depose him, but Inghean Dubh had him killed. On 6 January 1592 Hugh Roe escaped from Dublin to Fiach O'Byrne (qv), returning to Tír Conaill for his inauguration on 3 May. Aodh abdicated in favour of his son, dying of old age in 1600.
Aodh was an opportunist both as a soldier and a politician. However, either as a rebel or as lord of Tír Conaill, he was always too weak politically and militarily to deal with the combined challenges of the power of the O'Neills in Ulster, the extension of English control into north Connacht, and the strength of his rivals in Tír Conaill.