O'Shaughnessy was born on December 22, 1850 in Dublin, son of Thomas O'Shaughnessy and Mary Lopdell. He married Catherine Newman in 1879 and they had four children. He died at his home in Fitzwilliam Square, Dublin on March 7, 1933. Educated at Queens College Galway, he was called to the Irish Bar in 1874 and to the English Bar by Middle Temple in 1894.