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