McDonnell, Alexander (1829–1904), civil and locomotive engineer, was born in Dublin on 18 December 1829, third son amongst six sons and five daughters born to John MacDonnell (qv), an eminent Dublin doctor, and his wife Charity, daughter of the Rev. Robert Dobbs. His father and grandfather both used the form ‘MacDonnell’. After entering TCD (1 July 1847) McDonnell graduated with a BA in mathematics (1852) and an MA (1861). He served his three-year engineering apprenticeship under Charles Liddell of the firm Liddell & Gordon in Westminster, during which time he also completed a course of study at the Écoles des Ponts et Chaussées in Paris. With his training completed, he remained with the firm till 1858, when he accepted the post of resident and locomotive engineer in the construction of the Newport & Hereford Railway, Wales; on its completion, he continued as its engineer and locomotive superintendent (1858–61). In 1861 he joined the Danube & Black Sea Railway Co. and was sent to eastern Europe to organise its locomotive department (1861–3).
He returned to Dublin in 1864 as the Great Southern & Western Railway Co.'s locomotive engineer and superintendent of the locomotive department in Inchicore, a post he occupied for eighteen years (1864–82). Described as a ‘brilliant civil engineer’ (Ryan, 13), he equally proved to be an exceptional administrator: his meticulous and detailed book-keeping set a standard that was maintained throughout the history of the factory, and his thorough overhaul and upgrading of the entire manufacturing system rendered it more efficient and ultimately more productive. By 1877 the carriage and wagon departments were also placed under his command, effectively putting him in charge of Inchicore's entire operations, and under his supervision the departments were developed and expanded, by 1879 producing all the rolling stock needed for the various branches of the railway. As locomotive engineer he designed engines for passenger (including express) trains and was known for his sturdy cabs and innovative designs; he insisted that no materials go to waste, and many of his engines contained recycled parts from older machines. His most famous model was the 101 class, a goods engine that was eventually used on every branch and for every duty on the whole Irish system after the amalgamation of the railways in the twentieth century. So well conceived were his engines that by the 1990s nos 184 and 186 were still being used by the Railway Preservation Society of Ireland to pull trains and delight railway enthusiasts from around the world.
Part of McDonnell's accomplishment was his recognition that success depended on a happy and productive workforce; he proved himself a firm but sympathetic employer who won his workers' respect by understanding their needs and providing for their wants. His interest in their welfare made the GS & W Railway Co. a model for its time for employee treatment, and it was thanks to McDonnell that the Inchicore works had a proper dining room and a large reading room. By 1879 the factory was running like clockwork, and the production of locomotives, carriages, and wagons was so good that McDonnell threw a grand gala in recognition of everybody's hard work (8 August 1879); it was an entire day of events attended by over 2,000 employees and guests.
In order to meet a new challenge, he left Inchicore in 1882 to take up appointment as locomotive superintendent of the North Eastern Railway Co. in Gateshead, England. However, his work there was not as successful: several of his designs performed poorly and his sweeping changes, so successful in Inchicore, merely antagonised his new employees; as a result, he resigned in 1884. For the remainder of his career, he worked as a consultant, visiting Brazil twice and Australia once. He was elected a member of the Institution of Civil Engineers in London on 6 February 1872 and served as president of the ICEI in 1876. After a short illness he died in Holyhead, Wales, on 4 December 1904, while returning to Ireland. His portrait hangs in the Institution of Engineers of Ireland.
He married (1867) Isabelle Blanche, daughter of George St Leger Grenfell; they lived for a time at St John's, Islandbridge, Dublin, and had two sons, Ian Alaster (b. 1868) and James Riversdale (b. 1872), and a daughter, Marie Louise.