He was born in what is now the Park House pub in Lower Main Street in 1828. His father was a wealthy Dungarvan merchant. While studying for medicine in Dublin he became involved with the Young Irelanders. Because of his involvement in the Fenian Rising in 1867 he had the distinction of being the last man to be sentenced to be hanged under British law in Ireland. However this was commuted to penal servitude for life. He went to Paris with John O'Leary where he continued his studies in medicine. O'Brien went to America where he engaged in a prominent part in the American Civil War working as an assistant surgeon. Later elected M.P. for South Mayo and M.P. for Cork, he died on 28 May 1905.