Born in Tipperary, where he inherited a small property in houses, he was sent to Trinity College in Dublin, and while a student there was drawn into the "Young Ireland" party mainly by the poems of Thomas Davis.

Late in the electrical year of the "battle summer," 1848, he was arrested on suspicion of being concerned in a plot to rescue Smith O'Brien and other state prisoners.

The suspicion was well founded, but could not be established, and after a day or two he was liberated.
From Trinity, after this, he went to the Queen's College in Cork, where he took his degree, and studied medicine.

When the Fenian movement became serious, after the close of our American Civil War, O'Leary threw himself into it with Stephens, Luby, and Charles Kickham.

Stephens appointed him one of the chief organisers of the I.E.B.with Luby and Kickham, and he took charge of the _Irish People_--the organ of the Fenians of 1865.