O’Leary was born in Tipperary town on 23 July 1830.
In 1846, while convalescing at home after an attack of typhus he discovered the writings of Young Irelander Thomas Davis, underwent a political conversion experience, and became a reader of the Nation – a nationalist newspaper founded by Charles Gavan Duffy in 1840.
In November 1848 O'Leary was arrested with a group of Young Ireland sympathisers who had assembled at the ‘Wilderness’ outside Clonmel as part of an unsuccessful plan to free convicted Young Ireland leaders; he was detained for a few weeks and then released. In 1849 he took part in an abortive revolutionary conspiracy led by James Fintan Lalor, which led to sporadic attempts at a rising in Tipperary and Waterford.
O’Leary studied medicine at Queen’s College Cork and then at Queen’s College Galway. He remarked in his memoirs that bookstalls, bookshops, and books generally had taken up such a large proportion of his life that no account of it could be complete without mentioning the fact. His first publication was a letter in defence of the queen's colleges published in the Nation on 26 October 1850; Cardinal Cullen subsequently pointed to O'Leary as proof that the colleges encouraged irreligion and political subversion.
In 1858-59, while studying in Dublin, he was recruited by James Stephens for the latter’s new secret society- the Irish Republican Brotherhood (IRB). O'Leary became the chief financial agent of the new society and was sent to America on a fund-raising mission. In August 1863, Stephens recalled him to act as editor of the new Fenian newspaper, the Irish People, which commenced publication in November 1863.
O'Leary was one of a three-member committee authorised to act for Stephens as IRB leaders in his absence: the others were Thomas Clarke Luby and Charles Kickham. The discovery of Stephens's commission constituting this committee played a major role in O'Leary's prosecution for Fenian activities. O'Leary was arrested in the early morning of 16 September 1865. He was tried before a special commission on 1–7 December, found guilty of treason-felony, and sentenced to twenty years’ imprisonment.
O'Leary received a pardon in January 1871 on condition that he remained outside the UK for the duration of his original sentence. Most Fenian activists released on these conditions went to America, but O'Leary chose to settle in Paris, where he mixed with Irish and other revolutionary exiles.
O'Leary returned to Ireland in January 1885, settled in Rathmines with his sister Ellen (the poet) and mingled in literary and political circles. At the Dublin Contemporary Club he met W. B. Yeats, whose genius he recognised; throughout the late 1880s and 1890s he gave the young poet access to his bookshelves. For Yeats, O'Leary represented a powerful, idiosyncratically honest form of nationalism dissociated from clericalism, rhetoric, and mob politics.
His sister Ellen’s death in 1889 hit O’Leary hard and exacerbated a long-standing tendency to aimlessness, depression, and alcoholism. In his last years O'Leary was associated with Arthur Griffith, who regarded him as the elder statesman of Irish nationalism.
O'Leary died in Dublin 16 March 1907.Sources:
https://www.dib.ie/biography/oleary-john-a6861
