O'Donnell (Ó Domhnaill), An Calbhach (d. 1566), lord of Tír Conaill, was eldest son of Manus O’Donnell (qv), lord of Tír Conaill, and Joan, daughter of O’Reilly of Cavan. Little is known of his youth. He may have been among the sons of Manus defeated by their uncle, Aodh Buidhe, in 1531. On his inauguration (1537) Manus sought to restore the former O'Donnell hegemony in north Connacht and Ulster. During this process, An Calbhach appears doing his father's bidding by raiding the territory of Clanawley (Co. Fermanagh) and Ó Catháin in 1537.
During the late 1530s the O'Donnells allied themselves temporarily with the Geraldine League. Their membership of the league held in check the ambitions of their powerful neighbour, Con Bacach O’Neill (qv), lord of Tír Eóghain. After Lord Deputy Leonard Grey (qv) defeated them and O’Neill in 1539, the O'Donnells resumed amicable relations with the government. In the early 1540s An Calbhach clearly emerged as the heir of Manus. During 1542 he campaigned against his father's clients in north Connacht and within the Antrim glens. The trust Manus had in An Calbhach's abilities is evident when he dispatched him to Dublin to confirm his peace with the English. It is notable that the annalist mentions that An Calbhach also ratified his own peace with the administration, which indicates he had an independent power base within Tír Conaill by 1542. Splits in the dynasty surfaced in 1543, when the O'Donnell castle of Lifford was taken by rebels supported by An Calbhach's brother, Aodh mac Maghnusa O’Donnell (qv). In response An Calbhach attacked Lifford but was repulsed. O'Donnell weakness was evident when An Calbhach sought English aid to successfully retake Lifford in 1544.
Throughout 1544–5 An Calbhach's star continued its ascent with victories over O’Neill and O'Donnell rebels. After 1545 his relationship with Manus became acrimonious over the issue of succession. Manus was resolute that An Calbhach would wait, but his stance alienated his son. In 1547 An Calbhach revolted but was beaten by Manus near Ballybofey in the following year. A low-level war simmered between father and son till 1552. In 1555 An Calbhach took ship for Scotland, recruited MacDonnell mercenaries, and imported these, along with artillery, into Tír Conaill. Through skilful use of artillery he took his father's fortresses of Greencastle and Eanach in Inis Eóghain. Realising northern Tír Conaill was lost, Manus surrendered.
The usurper immediately had to contend with the rise of Seaán O’Neill (qv), lord of Tír Eóghain. Although O’Neill was married to An Calbhach's sister, Margaret, he was eager to be master of Ulster, and encouraged the ambitions of An Calbhach's brother, Aodh. In 1557 he invaded Tír Conaill. On hearing this, An Calbhach sought the advice of Manus, who told him to retreat before O’Neill and wait for an opportunity. While O’Neill's troops were feasting, An Calbhach launched a powerful counter-thrust which routed O’Neill's unprepared army.
Despite this victory, O’Neill's threat remained, and in 1560–61 An Calbhach became involved in a plot against him. Archibald Campbell, 5th earl of Argyll, offered the Dublin government an alliance of Campbells, MacDonnells, and O'Donnells that would combine with a government army to defeat O’Neill. This alliance would be conditional on the government's recognition of the MacDonnell settlements in the Antrim Glens. As part of the agreement An Calbhach married Catherine MacLean, Argyll's stepmother. O’Neill, however, struck first. On 14 May 1561 he captured An Calbhach and his new wife. An Calbhach was confined in Tir Eóghain and (by his own account) chained cruelly in a cage in O’Neill's bedroom, often while O’Neill was intimate with Catherine MacLean. O’Neill later shattered the alliance by routing the rearguard of the retreating government army in July 1561. Then he installed An Calbhach's brother Aodh as lord of Tír Conaill. Early in 1564 O’Neill released An Calbhach in exchange for the surrender of Lifford and the promise of a large ransom. After his release, the exiled and almost destitute An Calbhach went first to Dublin – where he was reduced to tears by the indifference of officials – then to Whitehall, where Elizabeth I listened sympathetically to his plight and promised him help. In September 1565 Lord Deputy Sir Henry Sidney (qv) reinstalled An Calbhach as lord of Tír Conaill. The luckless An Calbhach lived briefly to enjoy his restoration, dying from a fit while riding on 26 October 1566.
Although An Calbhach was a talented and active soldier, he did not have the ability of his father or grandfather. He did, however, usurp a lordship already in decline and was confronted with problems never faced by his precedessors. This inherited weakness, combined with the rise of Seaán O’Neill, destroyed any prospect of An Calbhach's restoring Tír Conaill's fortunes.