The son of William P. Cairnes of Stameen, Drogheda, William Jameson Cairnes was educated at Rugby and Cambridge. An infantry officer with the Leinster Regiment, he joined the Royal Flying Corps in Egypt and became a Flying Officer on 6 November 1916. He was promoted to Captain on 20 December 1916. After scoring 4 victories with 19 Squadron, Cairnes was appointed Flight Commander on 1 February 1918. In the spring of 1918, he scored 2 more victories with 74 Squadron before he was killed in action, NE of Estaires. His S.E.5 was shot down by Paul Billik of Jasta 52. Ira Jones witnessed the event and reported Cairnes' plane went down after losing a wing.