He inherited the barony when his father was killed in action in Italy during the Second World War.
[2] His mother Ann (a noted society hostess and the grand-daughter of the 11th Earl of Wemyss) remarried, firstly in 1945 to the press magnate Esmond Harmsworth, 2nd Viscount Rothermere, and secondly (after Rothermere divorced her in 1951) to the writer Ian Fleming (died 1964) in 1952, as well as having affairs with the Labour politicians Hugh Gaitskell and Roy Jenkins.
[4] The following year, he was transferred into the North Irish Horse, a unit in the Territorial Army, with the same rank;[5] he was promoted to Lieutenant in 1956 (with seniority from 1967),[6] Captain in 1961,[7] and temporary Major in 1964 (which was made substantive in 1967).
[9] Between 1986 and 1991, he was Honorary Colonel D (North Irish Horse) Squadron, The Royal Yeomanry, in 1986;[10][11] and between 1988 and 1993, he was Honorary Colonel 69 (North Irish Horse) Signal Squadron, 32 (Scottish) Signal Regiment (Volunteers).
[1] O'Neill was appointed a Knight Commander of the Royal Victorian Order (KCVO) in the 2009 New Year Honours.