MacDonnell, Alexander (1615–99), 3rd earl of Antrim , politician and soldier, was the second son of Sir Randal MacDonnell, 1st earl of Antrim (qv), and Alice, daughter of Hugh O'Neill (qv), earl of Tyrone. Alexander spent much of his childhood at court in England, but on the death of his father (1638) he inherited the barony and castle of Glenarm, Co. Antrim. MacDonnell handed the lands over to his elder brother Randal (qv), 2nd earl of Antrim, for ‘a certain sum of money yearly for his maintenance’ (quoted in Ohlmeyer, 32). He spent the next three years on the Continent (though little is known of his activities there) before returning to Ireland shortly before the outbreak of the Ulster rising in October 1641. MacDonnell travelled to Edinburgh and London before the end of the year, but failed to receive a commission to serve in the king's forces, probably due to his catholicism. In 1642 he created his own regiment, which subsequently became an integral part of the Ulster army of Owen Roe O'Neill (qv). MacDonnell organised the winter raid to the Scottish Highlands (1643) by Alasdair MacDonnell (qv), and fought at the battle of Benburb (June 1646).
Despite his military exploits, however, MacDonnell was essentially a politician, who worked closely with his brother throughout the 1640s. He sat on every confederate supreme council from 1644 until the dissolution of the association (January 1649). In early 1644 he travelled to Oxford as part of a confederate delegation, to seek an accommodation with Charles I. On his return to Ireland in July, the general assembly in Kilkenny appointed MacDonnell to the committee of treaty, charged with negotiating a peace deal with the royalist lord lieutenant, James Butler (qv), marquis of Ormond. MacDonnell played an active role in the ensuing talks, and in August 1645 he also signed the secret Glamorgan treaty, which promised major religious concessions from the king. He supported the peace treaty between the confederates and Ormond, proclaimed in August 1646, despite the opposition of the papal nuncio, GianBattista Rinuccini (qv). Shortly afterwards, however, displaying a political pragmatism often associated with his elder brother, MacDonnell switched sides and agreed to serve on the new supreme council established by Rinuccini. None the less, he remained sympathetic towards the peace faction, becoming involved in a number of anti-clerical plots during the course of 1647. MacDonnell withdrew his regiment from the Ulster army, and the following year openly sided with the nuncio's enemies. At the final general assembly, he supported the peace deal signed with Ormond on 17 January 1649. After O'Neill's death (November 1649), MacDonnell returned to the Ulster army, serving under Heber MacMahon (qv), bishop of Clogher. Captured by Sir Theophilus Jones (qv) at Tecroghan in 1651, he spent a number of years as a prisoner in London. The new Cromwellian regime declared his estates forfeited and MacDonnell accepted 7,000 transplantation acres in Co. Galway. According to the 1659 poll tax, he had moved to the town of Belfast, close to the family's former estates. Restored to his lands, along with his brother, by the 1665 act of explanation, he made Glenarm castle his main residence for the next twenty years.
In 1665 he married Elizabeth, daughter of Arthur Annesley, 1st earl of Anglesey (qv), but she died childless four years later. MacDonnell then married Helena, daughter of Sir John Burke of Derrymaclaghtny, Co. Galway. He succeeded to the title of earl of Antrim on the death of his brother in 1683, and played an active role in the subsequent political and military upheavals in Ireland. Appointed to the privy council, and as lord lieutenant of Co. Antrim, by Richard Talbot (qv), earl of Tyrconnell, Antrim raised a regiment in 1688 to support James II (qv). He should not be confused with his namesake, Col. Alexander MacDonnell, who was subsequently dismissed as governor of Galway. When he was sent to garrison Derry (December 1688), the apprentice boys, in a dramatic act of defiance, shut the city gates as he approached, forcing him to withdraw. His regiment fought at the Boyne (1690) and Aughrim (1691), although it is unclear if the aged earl accompanied his troops into battle.
Antrim attended the Jacobite parliament in Dublin in 1689, sitting in the house of lords, the only survivor from the confederate assemblies of the 1640s. After the treaty of Limerick (1691) Antrim, despite his attainder, was the only major catholic landowner not to go into exile. He spent his last years in London, using his extensive contacts to plead successfully for a full recovery of his estates. He also acted as unofficial agent for the few remaining catholic landowning families in Ireland. Antrim died in Holywell, Wales, in June 1699, and was succeeded in the title by his only legitimate son, Randal.