Born in Kill, Co Kildare, his family moved to Dublin where he was educated by the Christian Brothers.
Influenced by political activists he met at evening classes, he joined the Fenians security society in 1861.
He settled in Naas, Co Kildare, before he was recruited by Fenian leader James Stephens (not to be confused with the writer) to recruit Irishmen from British regiments.
He was arrested in February 1866 and sentenced to 15 years penal servitude but was released in 1871 and emigrated to the United States. There, he joined Clan na Gael (founded in 1867) which under his leadership became an influential Irish-American nationalist organization.
One of his most spectacular coups was the daring rescue of six Fenians from penal servitude in Fremantle, Australia in 1876 by means of the whaler Catalpa. He edited a successful paper, the Gaelic American, for which Tom Clarke also worked.
Clan na Gael supported the Irish Volunteers with funding and of arms and it is reported to have contributed $100,000 towards the Rising up to 1916. Although in his 70s, he was prepared to journey home for the Rising — of which he was made aware — but could not get travel documents in time.