O'Leary represented central Dublin in Dail Eireann, the Irish parliament, from 1965 to 1987, then qualified as a lawyer and practiced in his native Cork.
He was appointed a judge of the District Court, Ireland's lowest tier of courts, in 1997 and retired from that post on Monday. He drowned Thursday while swimming in a pool, colleagues said.
O'Leary had studied at University College, Cork, and at Columbia University in New York City, before beginning his professional life as an Irish Transport and General Workers Union organizer.
After his election to the Dail, he sought to steer Labour leftwards and away from Ireland's dominant conservative parties, Fianna Fail and Fine Gael, but eventually recognized that Labour would need to cooperate with one of them to form a government.
He served as labor minister in a Fine Gael-Labour coalition government from 1973 to 1977, when he narrowly lost an election to become Labour leader. O'Leary ascended to the post in 1981 when his longtime rival, Frank Cluskey, lost his own parliamentary seat, according to the AP.
At that time he led Labour into a second coalition with Fine Gael and became the government's deputy leader and energy minister. But he resigned as Labour leader when that government collapsed a year later - and, in a reflection of his increasing bitterness toward the party, jumped ship to Fine Gael. He served as a Fine Gael lawmaker from 1982 to 1987, when he failed to win re-election.
Prime Minister Bertie Ahern, the Fianna Fail leader who also represents central Dublin, praised O'Leary as "one of the wittiest and most entertaining men one could meet, and he enjoyed a considerable gift as a mimic."
He was survived by his wife, Mary. Funeral arrangements were not announced.