Born at Nenagh, Ireland, the son of journalist John O'Shea, in 1856 he was sent to study medicine at the Catholic University of Ireland in Dublin. Later that year he journeyed to London where he sought work as a journalist. He left to serve in Pope Pius IX's Irish battalion. During the 1860 siege of Ancona, he reported on the conflict for a newspaper in America. Following his military service for the Papacy, he was hired as a correspondent by the New York Herald and reported on the Austro-Prussian War in 1866.