D'Alton, John (1792–1867), historian and genealogist, was born 20 June 1792 at Bessville, Co. Westmeath, first son of William D'Alton and his wife Elizabeth Leyne. He was educated privately by the Rev. Joseph Hutton before entering TCD (1806). Deciding on a legal career, he gained admittance to the King's Inns, Dublin, in early 1809. Graduating BA (Dubl.) in 1811, he was admitted to the Middle Temple in May of the same year and was called to the Irish bar in 1813. He began his practice on the Connacht circuit and was especially sought after in cases where a knowledge of genealogy was required. In 1835 he became commissioner of the Irish Loan Board.
While he maintained his legal practice, it was in the area of historical research that he achieved renown. In 1827 he was awarded the RIA's Cunningham medal for the best essay on the political and social history of the early Christian period. Three years later he won another academy prize for his account of the reign of Henry II in Ireland. His relationship with the RIA was not always cordial, however, and throughout the 1830s he opposed the academy's plans to publish a series of historical memoirs, as he was planning such publications himself. In January 1833 he began to write for the Irish Penny Magazine, a journal established by Samuel Lover (qv), and contributed many learned historical articles. The journal was a failure and was discontinued in January 1834. Elected an honorary member of the RIA in December 1835, D'Alton also contributed a number of articles to the Gentleman's Magazine. In 1838 he was elected a corresponding member of the Society of Antiquaries of Scotland. A prolific writer, during the course of his career he published a vast number of works including Memoirs of the archbishops of Dublin (1838), History of the county of Dublin (1838), and Annals of Boyle (1845). His most valuable work was undoubtedly King James's Irish army list, 1689 (1855), which contained a wealth of biographical information on the officers included, and was subsequently enlarged and republished (1860) in separate volumes covering the cavalry and infantry arms. It remains a major reference work for historians of the period. D'Alton died 20 January 1867 at his Dublin residence, 48 Summer Hill, and was buried in Glasnevin cemetery.
He married (1818) Catherine Philips of Clonmore, Co. Mayo; they had several children. The contents of D'Alton's library, which included some seventeenth- and eighteenth-century Irish manuscripts, became the focus of much attention in the historical community after his death. The library was eventually broken up but there are large collections of D'Alton papers, including unpublished manuscripts, in the University of Chicago and the NLI in Dublin.