Cornelius O'Leary (30 September 1927 – 7 September 2006) was an Irish historian and political scientist.

O'Leary was born in Limerick but was raised in Cork,^[1] where he attended University College Cork, gaining a first-class honours degree in history and Latin in 1949.^[2] He subsequently studied for a DPhil at Nuffield College, Oxford as the first student to be supervised by the psephologist David Butler.^[3] While researching his PhD, he worked at a number of secondary schools in London.

His thesis was published as The Elimination of Corrupt Practices in British Elections, 1868-1911 (Clarendon Press, 1962) and in the same year he was appointed lecturer at Queen's University Belfast in Northern Ireland.^[2] He was made professor of political science in 1979, having been controversially denied such a post previously.^[2] He was the first Catholic to hold such a chair at the university^[2][4] and explained that, when he was appointed in 1960, he saw Queen's as part of the Unionist establishment.^[3] According to an obituary written by Bernard Crick, O'Leary suffered from alcoholism, which resulting in him often being absent from the university and colleagues having to cover for him.^[2] At one point he lived in hotels and lodging houses rather than at a fixed address.^[2]

Aside from his PhD thesis, O'Leary's main publications were Belfast: Approach to Crisis. A Study of Belfast Politics, 1613-1970 (with Ian Budge, Macmillan, 1973), The Northern Ireland Assembly, 1982-1986: A Constitutional Experiment (with Sydney Elliott and R.A. Wilford, Hurst, 1988) and Controversial Issues in Anglo-Irish Relations, 1910-1921 (with Patrick Maume, Four Courts, 2004). He had planned a work on 20th century Irish politics but this never materialised.^[2]

Although from the Republic of Ireland, O'Leary developed an interest in Unionism, and was an advisor to the Ulster Defence Association.^[3] He subsequently wrote a paper on Northern Irish independence at the request of John McMichael.^[3] During the 1960s and 1970s he was a regular commentator on Northern Ireland in the media.^[5] In his later life, he served as the vice-chairman of the anti-abortion campaign surrounding the Eighth Amendment of the Constitution of Ireland,^[3] which introduced a constitutional ban on abortion. This brought him into opposition with many feminists.^[2]

O'Leary died on 7 September 2006^[2] after a short illness.^[3] He is buried at Timoleague Abbey Cemetery in County Cork.^[3]