 Harris is believed to have been born in Athlone to Peter and Ann Harris, but spent most of his adult life in Ballinasloe, where he worked as a building contractor.His grandfather, Peter Harris, was executed in Monasterevin by the British forces during the Irish Rebellion of 1798.He was involved in all the major 19th-century political movements of Irish nationalism, and was in the turn a Repealer, a Young Irelander, before moving to join the Fenians, where he became the main Fenian representative in East Galway and South Roscommon.