Hennessy, William Maunsell (Ó hAonghusa, Liam M.) (1829?–1889), Irish-language scholar, was born in 1829 or possibly a year earlier, in Castlegregory, Co. Kerry; nothing is known of his parents. He was educated at private schools and spent his early youth with his uncle, Dr Finn. His education was probably largely classical in content and may also have included French and German. After leaving school he lived for a number of years in the USA before returning to Ireland to work as a journalist. From 1853 to 1856 he was on the staff of the Nation. In 1856 he obtained a post in the Office of Inspectors of Lunatic Asylums, where he worked until 1868, when he was appointed first-class clerk in the Public Record Office of Ireland. His extensive knowledge of Irish proved of great benefit to him in this post, and after the death (1886) of Sir Samuel Ferguson (qv) he was made assistant deputy keeper, an appointment he retained until his death. On his advice, an elementary knowledge of the Irish language became necessary for entry to a post in the PRO in Dublin.
Hennessy's greatest devotion was to the study of Irish manuscript literature. He was regarded as one of the greatest native Celtic scholars of his time and a distinguished successor to Eugene O'Curry (qv) and John O'Donovan (qv). His work combined a familiarity with European Celtic studies and a thorough knowledge of the Irish native tradition. Elected to the RIA in 1865, he served as Todd professor there (1882–4). His publications include a revision of James Graves's (qv) The pedigree of the White Knight (1856), Chronicum Scotorum (1866), The life of St Patrick with the tripartite life (1870), The annals of Loch Cé (2 vols, 1871), The book of Fenagh (1875), The annals of Ulster (vol. i, 1887) and the posthumous Mesca Ulad (1889). In addition, he was the first to bring the Middle Irish parody ‘Aislinge Meic Conglinne’ to the public's attention, through a translation he published in Fraser's Magazine in September 1873. He contributed to the Ossian controversy in an essay entitled ‘Ossian and Ossianic literature’ which appeared in The Academy (1 Aug. 1871, 365–67 and 15 Aug. 1871, 390–94), and also published articles in continental journals such as Revue Celtique, Kuhn's Zeitschrift and Beiträge für vergleichende Sprachforschung. Interested in Irish topography and antiquities, he published an essay entitled ‘The Curragh of Kildare’ in the Proceedings of the Royal Irish Academy, ix (1864–6), 343–55. He also revised the Irish texts in the third edition of John O'Daly's (qv) Poets and poetry of Munster (1883).
He was involved in the founding of the Society for the Preservation of the Irish Language in 1876, and served as a council member for many years. Although a native Irish-speaker, his attitude to the modern language was reportedly ambivalent: John Fleming (qv), writing in Irisleabhar na Gaedhilge (1889), claimed that Hennessy had an ‘aversion’ to the modern language. This, Fleming believed, stemmed from the particular dislike Hennessy had for many of the people involved in its preservation. Hennessy died at home at 71 Pembroke Road, Ballsbridge, Dublin, on 13 January 1889 and is buried in Glasnevin cemetery.
His wife (d. 1885) (name unknown) and a daughter predeceased him. His son, William Charles Hennessy (1857–98), was a barrister and writer who contributed to the Nation under the pseudonym ‘Seehaitch’ and wrote several pantomimes.