Born and educated in Ireland, O’Brien emigrated to the U.S. at age 24 in 1852. There he became a leader among the New York Bohemians as he began to publish fiction and poetry in various periodicals. He also wrote a number of plays which were very popular at the time, and was an accomplished short story writer who helped develop the American literary magazine, though today his work is little-known. O’Brien volunteered to fight with Union forces in the Civil War, and in 1862 he died of complications from a wound suffered in battle.