Born at Ballybeg or Clonee, Ferns, County Wexford, where his father was an employee at the Sinnott estate, he later became an Irish republican. Ultimately, he served as getaway driver during the assassination of Permanent Under Secretary Thomas Henry Burke and Lord Frederick Cavendish in Phoenix Park.
He was later tried and found not guilty of the murders but in a retrial in May 1883, was convicted of conspiracy and accessory to murder and sentenced to penal servitude for life. He was released from prison in 1899 and visited the United States until he was deported back to Ireland in 1900.
He is mentioned in the Irish folk song, "Monto (Take Her Up to Monto)", written by George Desmond Hodnett and popularised by The Dubliners. He is also mentioned in Ulysses by James Joyce, in Chapter 07 under the headline The Greater Gallaher. To quote the line said by Myles Crawford "Time Kelly, oe Kavanagh I mean, Joe Brady and the rest of them. Where Skin-the-goat drove the car. Whole route, see?"
FitzHarris died on 7 September 1910, in the South Dublin Union workhouse where he had been living in penury. He is buried in Glasnevin Cemetery. The following day the London Times honoured this simple but courageous Wexford man with an obituary. His death certificate noted that he was married but little is known of his wife or family.