O'Neill was the second but eldest surviving son of Edward O'Neill, 2nd Baron O'Neill, and his wife Lady Louisa Katherine Emma (née Cochrane). Hugh O'Neill, later Baron Rathcavan, was his younger brother.
O'Neill fought in the First World War as a Captain in the 2nd Life Guards and was killed in action at Klein Zillebeke ridge, 6 November 1914, aged 38, the first MP to be killed in the conflict. His brother Hugh succeeded him as MP for Mid-Antrim.
O'Neill married, at St Paul's Church, Knightsbridge, on 21 January 1902, Lady Annabel Crewe-Milnes, daughter of Robert Crewe-Milnes, 1st Marquess of Crewe. The Archbishop of Armagh, Primate of All Ireland, performed the ceremony. Lady Annabel O'Neill later remarried and died in 1948.
- Kidd, Charles, Williamson, David (editors). Debrett's Peerage and Baronetage (1990 edition). New York: St Martin's Press, 1990,[page needed]
- Leigh Rayment's Historical List of MPs [self-published source][better source needed]
- Lundy, Darryl. "FAQ". The Peerage.[unreliable source]
- Hansard 1803–2005: contributions in Parliament by Arthur O'Neill
|Parliament of the United Kingdom|
Robert Torrens O'Neill
|Member of Parliament for Mid Antrim
January 1910 – 1914