O'Neill, Thomas Patrick (1921–96), historian, was born 1 November 1921 in Ballon, Co. Carlow, the son of Thomas O'Neill, a farmer, and his wife, Anna Maria (née Murphy); he had at least two brothers. He was educated in the local national school, at Knockbeg College, and at UCD, where he completed his MA dissertation in 1946. A shortened version was published as a chapter in R. D. Edwards and T. D. Williams (ed.), The great famine (1956). Space considerations compelled the omission of work on private relief efforts, which O'Neill published in journal articles; he was one of the few professional historians of his generation to devote a significant proportion of his research career to the famine. He went beyond the published sources to make pioneering use of archival material; although his chapter is often criticised for focusing on the administrators of famine relief rather than its recipients (and O'Neill himself declared in later life that if he wrote it again he would be more critical of the British civil service for their handling of the catastrophe), he laid the foundations for most subsequent treatment of the relief effort. O'Neill's contributions were not confined to his own chapter; he proposed the overall format of the volume and devised the questionnaire sent out by the Irish Folklore Commission (which provided the basis for the chapter on folk memories of the famine by Roger McHugh (qv)) and he also prepared some statistically based maps which were incorporated in the volume.
In 1947 O'Neill was appointed assistant keeper of printed books at the NLI; his experience of the library, at a time when its manuscripts were much less well catalogued than they have been latterly, laid the foundation for his remarkable command of the source materials for modern Irish history. Among his other civil service duties he wrote propaganda for the Anti-Partition Campaign under the pen-name Lionel Thomas. In 1958 the Library Association of Ireland, of which he was a fellow, published his pamphlet Sources of Irish local history, collecting eight articles published in the association's journal An Leabharlann on different types of sources, for the benefit of librarians faced with guidance-seeking local historians. This pamphlet long remained an indispensable research tool for local historians. (It is described as ‘first series’ and states that O'Neill intended to write further articles on related topics for An Leabharlann, but only one more article appeared.)
In 1962 O'Neill published an Irish-language biography of James Fintan Lalor (qv), Fiontán Ó Leathlobhair (an English-language translation was published in 2003). Although O'Neill could speak and write Irish it was very much his second language, and he acknowledged the assistance of Donncha Ó Céileachair (qv) in bringing the work up to publishable standard (just as Padraig Ó Fiannachta's role in their jointly authored Irish-language biography of Éamon de Valera (qv) was primarily as translator and polisher of O'Neill's text). The Lalor biography drew on previously unused newspaper and archive sources, and has remained the standard work on its subject; among other discoveries O'Neill found that Lalor had written to Sir Robert Peel (qv) expressing hostility to the repeal movement and proposing land reform as a means of killing it off. (O'Neill was horrified when some friends advised him to suppress this material on supposedly patriotic grounds.)
The Lalor biography led to the commissioning of the work for which O'Neill is best remembered. He was on friendly terms with Frank Gallagher (qv), who was also employed by the NLI and was working on the authorised biography of de Valera (and, aided by O'Neill, on a projected dictionary of Irish biography). When Gallagher died in July 1962, leaving only some chapters on the Anglo-Irish treaty negotiations (edited for publication in book form in 1965 with an introduction by O'Neill), de Valera approached O'Neill to take up the project. In 1963 a contract was signed and O'Neill was formally seconded to the presidential staff to undertake the task; his work involved long and regular interviews with de Valera, and for a time he actually lived in Áras an Uachtaráin for greater ease of access to de Valera's private papers. O'Neill developed a fondness for his subject and remained in close contact with the de Valera family for the remainder of his life. During this period he also acted as historical adviser on the documentary films Mise Éire and Saoirse? (about the 1916 rising and war of independence respectively).
An Irish-language version of the biography was published in two volumes by Cló Morainn in 1968 and 1970, with an t-Athair Pádraig Ó Fiannachta as co-author; in 1970 an English-language text was published by Hutchinson as by O'Neill and Lord Longford (Frank Pakenham (qv)). Longford was recruited because the publishers believed a high-profile co-author would make the volume more saleable; he compressed O'Neill's original text (hence the Irish-language version is regarded as superior) and added some material on de Valera's religious faith. The text is fluently written and probably more professional than a work by Gallagher would have been; it preserves much valuable oral material about its subject's early life and his children's memories, but its status as a closely supervised official biography (de Valera read the manuscript thirty times) means that its account of de Valera's career is fundamentally incurious about the viewpoints of his opponents. Much subsequent scholarship on de Valera has reacted against it, and O'Neill's failure to complete a work of comparable scale on any other topic may lead those unacquainted with his overall career to suspect him of being a mere court historian.
In 1967 O'Neill was appointed lecturer in history at UCG, where he subsequently became associate professor. He was popular with students, who contrasted his fluent and spontaneous lecturing style with that of G. A. Hayes-McCoy (qv) (unkindly described as speaking as if his lectures had been composed in the nineteenth century). O'Neill took a leading role in the growth of extension lecturing from about 1970, addressing local history societies in such venues as Tiffy Winkle's Bar in Kinvara. O'Neill attached great importance to local historians, believing they often turned up information which would otherwise remain unknown to professional historians, and his willingness to treat laypeople with respect and answer their queries was credited with doing much to bridge the gap between town and gown locally. Even after returning to Dublin on his retirement in 1987, he kept in touch with Galway through his continuing involvement with local journalism and the Galway Family History Project.
O'Neill's major contribution to the civic culture of Galway was his suggestion that in 1984 there should be a large-scale commemoration of the 500th anniversary of Galway's first charter. (He had discovered the original document in the British Library in 1947.) He accompanied the mayor, Michael Leahy, and the city manager on several tours of America publicising the occasion; Leahy stated that one businessman estimated the publicity as worth $18 million to the city, and several other Irish cities discovered anniversaries of their own and imitated the celebrations. As part of the quincentennial commemorations O'Neill published a column on Galway history in the weekly Galway Advertiser. This was originally intended to cease at the end of 1984, but proved so popular that O'Neill was persuaded to continue it and did so until his death.
In retirement O'Neill was often to be found in the National Library, and remained active in many historical bodies, including the RSAI, the Carlow County Heritage Society (of which he was a founder member), and the Catholic Record Society of Ireland. While he never completed his magnum opus on the Registry of Deeds, his 1983 lecture (based on this research) on the impact of the penal laws on Dublin property ownership was awarded the medal of the Old Dublin Society. Although he did not disguise his nationalist views (he privately told Terry de Valera (1922–2007) that if ‘revisionist’ historians wanted ‘that sort of thing, why don't they call it what it is, fiction, and write novels’), his charm, sense of humour, and willingness to share information won him friends of all shades of opinion.
O'Neill died of cancer 1 March 1996 in St Vincent's Hospital, Dublin, after a long illness. He bequeathed his papers to the NLI. His failure to complete a major academic work disguises his considerable importance in building and sustaining the infrastructure of Irish historical research at a time when the profession was underdeveloped and sources very often inaccessible; such contributions are as intangible for future and better funded generations as the impact of his generous personality.
O'Neill was married twice; after the death of his first wife, Máiread O'Connor, he married Máire O'Kelly, a solicitor and former secretary to de Valera, and a talented historian in her own right. He had three sons and three daughters.