O'Reilly was a Fenian who enlisted in the British army in order to win support for the Irish cause. Upon discovery of his sedition he was court martialled (charged with withholding knowledge of an intended mutiny), found guilty and sentenced to death. His sentence was later commuted to life imprisonment and then to twenty years penal servitude. He was transported to Western Australia in 1868 on the last convict ship to be sent to the Australian colonies. He escaped imprisonment in Western Australia and departed for the USA on an American whaling vessel in 1869. O'Reilly took up residence in Boston where he became a journalist, an editor and a writer. He was part-owner of Catholic newspaper The Pilot which published his novel Moondyne and also wrote a treatise entitled Ethics of Boxing and Manly Sport (1888). O'Reilly continued to support the Irish cause throughout his life, moderating his views to favour constitutional reform rather than physical force. In 1876 he arranged the escape of Irish prisoners from Fremantle gaol.