Born and raised in Rosscarbery, West Cork of County Cork in the South of Ireland during the Great Irish Famine, O'Donovan founded the Phoenix National and Literary Society and dedicated his life to working towards the establishment of an independent Irish Republic. He joined the Irish Republican Brotherhood and after fleeing to the United States as part of the Cuba Five, he joined Irish revolutionary organisations there, beyond the reach of the British Empire. He was a pioneer in physical force Irish republicanism utilising dynamite in a campaign of asymmetrical warfare, hitting the British Empire on its home territory, primarily London.