MacDonnell, Sir Alexander (1794–1875), resident commissioner of national education, was born in Belfast, elder of two sons of James MacDonnell (qv), MD, of Belfast and Murlough, Co. Antrim, and his wife Elizabeth, daughter of J. Clarke, of Belfast. Winning a king's scholarship in 1809, MacDonnell received his early education at Westminster School (1809–13) before entering Christ Church, Oxford (1 June 1813), where he was an undergraduate, and then student (fellow) until 1826. He was a brilliant scholar and won four prominent university prizes for his English and Latin poetry and essay composition; his prize-winning poem ‘The horses of Lysippus’ was published in 1823. He graduated BA (1816) and MA (1820) and entered Lincoln's Inn 22 May 1817. After being called to the English bar (22 November 1824), he prastised on the midland circuit, attending the Leicester and Northampton sessions, during which time he also served as a commissioner of inquiry into public charities. He was a passionate, caring and extremely sensitive man, so much so that once, while pleading a case before the house of lords, he broke down in tears. Utterly mortified, he renounced the law and fled to Dublin, where he was appointed chief clerk (1835–9) in the office of Lord Morpeth (qv), chief secretary for Ireland.
In 1839 MacDonnell was appointed resident commissioner of the Board of National Education of Ireland at a salary of £1,000 a year. In this capacity (1839–71), he was responsible for the everyday operation of the board, including attending all subcommittee meetings, handling day-to-day correspondence, and directing the board's entire civil service. In 1854–5 he proved instrumental in the revision and clarification of the education code. He was noted for his scrupulous integrity and for the honourable manner in which he discharged his duties: throughout his many years of service he succeeded in improving the Irish education system without offending or attracting the wrath of any of the main religious groups. Although a devoted member of the Church of Ireland, MacDonnell believed in freedom of religion, so much so that in 1866 he allowed religious instruction in certain schools, providing that the teacher and students were of the same faith. Due to his general knowledge and practical experience, he was appointed to the Irish privy council in 1846. After serving as resident commissioner for thirty-two years, his age began to have an effect and he chose to retire in December 1871. In honour of his retirement and his many years of service to the nation, Queen Victoria awarded him a baronetcy on 20 January 1872. He died at his residence at 32 Upper Fitzwilliam St., Dublin, after a short illness and was buried at Kilsharvan, near Drogheda. He was fondly remembered as a frank and enthusiastic person with a firm character and a charming manner who was considerate and courteous to all.
He married (1826) Barbara (d. 6 April 1865), eldest daughter of Hugh Montgomery of Benvarden, Co. Antrim; they had no children. His brother John MacDonnell (qv) was an eminent Irish doctor and a pioneer in surgical anaesthesia. Sir Thomas Farrell (qv) sculpted a 12-ft (3.66 m) marble statue of Sir Alexander, which was erected (August 1878) in his honour in the grounds adjoining Tyrone House, Marlborough St., Dublin. Some of his papers can be found among the Carlisle papers at Castle Howard, Yorkshire.