Although qualified as a civil engineer, he is best remembered for Silva Gadelica (two vols, 1892), a collection of tales from ancient Irish manuscripts. He was a friend of antiquaries John O'Donovan and Eugene O'Curry. In 1853 he became a founding member of the Ossianic Society. He would later become its president in 1855. In 1857 he moved to the United States of America where he remained for 30 years. In 1901 he contributed an essay on Anglo-Irish Aristocracy to a collection entitled Ideals in Ireland edited by Lady Augusta Gregory.
He died in England in 1915. His Catalogue of the Irish Manuscripts in the British Museum was unfinished on his death and was completed by Robin Flower.