John O'Leary (1830 - 1907) was an Irish poet and fenian. He studied both law and medicine but did not take a degree and for his political beliefs he was imprisoned in England during the nineteenth century.

John O'Leary (aka Johnny Leary) (born 1st September 1894–21st June 1959) was a Irish Labour Party party politician who served in the Oireachtas for nearly twenty years, first as TD for Wexford and then as a Senator.

John O'Leary (born 1933) was a Irish Fianna Fáil party politician and who served for more than thirty years as TD for Kerry South.

John O'Leary is a Wicklow born Irish Gaelic footballer who played inter-county football for Dublin. John won the All-Ireland Senior Football Championship with Dublin on two occasions, in 1995 and 1983. He captained the 1995 side that beat Tyrone.

John O'Leary (1947-2 April 2005) served as mayor of Portland, Maine, and as U.S. ambassador to Chile under President Bill Clinton.