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 joined the British Army in 1897. He saw active service in South Africa between 1899 and 1900, during the Second Boer War. He was awarded the Queen's South Africa Medal with three clasps. In January 1902, he was temporary appointed Adjutant to the 2nd Regiment of Life Guards.
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. He is commemorated on the Menin Gate memorial. O'Neill is also commemorated on Panel 8 of the Parliamentary War Memorial in Westminster Hall, one of 22 MPs that died during World War I to be named on that memorial. O'Neill is one of 19 MPs who fell in the war who are commemorated by heraldic shields in the Commons Chamber. A further act of commemoration came with the unveiling in 1932 of a manuscript-style illuminated book of remembrance for the House of Commons, which includes a short biographical account of the life and death of O'Neill.
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.
O'Neill and his wife had five children; three boys and two girls. Their youngest child, Terence, was less than two months old at the time of his father's death. Their eldest son Shane succeeded his grandfather in the barony in 1928, while their third son Terence O'Neill was Prime Minister of Northern Ireland between 1963 and 1969.
- Hon. Sibyl O'Neill (1902 to 1946), married Lt Col Edward North Buxton, MC in 1924
- Hon. Mary Louisa Hermione O'Neill (born 1905), married Lt Col Derek Ernest Frederick Orby Gascoigne in 1934
- Shane O'Neill, 3rd Baron O'Neill (1907 to 1944 KIA)
- Hon. Brian Arthur O'Neill (1911 to 1940 KIA)
- Terence O'Neill, Baron O'Neill of the Maine (1914 to 1990), Prime Minister of Northern Ireland
- "O'NEILL, Hon. Arthur Edward Bruce". Who Was Who. A & C Black. April 2014. Retrieved 26 November 2014.
- "no. 27403". The London Gazette. 4 February 1902. p. 716.
- "Casualty Details: O'Neill, The Hon. Arthur Edward Bruce". Commonwealth War Graves Commission. Retrieved 30 August 2016.
- "Recording Angel memorial Panel 8". Recording Angel memorial, Westminster Hall. UK Parliament (www.parliament.uk). Retrieved 31 August 2016.
- "List of names on the Recording Angel memorial, Westminster Hall" (pdf). Recording Angel memorial, Westminster Hall. UK Parliament (www.parliament.uk). Retrieved 31 August 2016.
- "O'Neill". Heraldic shields to MPs, First World War. UK Parliament (www.parliament.uk). Retrieved 1 September 2016.
- "House of Commons War Memorial: Final Volumes Unveiled by The Speaker". The Times (46050). London. 6 February 1932. p. 7.
- Moss-Blundell, Edward Whitaker, ed. (1931). The House of Commons Book of Remembrance 1914–1918. E. Mathews & Marrot.
- "Court circular". The Times (36671). London. 22 January 1902. p. 10.
- Mulholland, Marc (September 2012). "O'Neill, Terence Marne, Baron O'Neill of the Maine (1914–1990)". Oxford Dictionary of National Biography. Oxford University Press. Retrieved 26 November 2014.
- "no. 33567". The London Gazette. 3 January 1930. p. 45. Retrieved 27 November 2014.
- Lundy, Darryl. "Captain Hon. Arthur Edward Bruce O'Neill". The Peerage. Retrieved 27 November 2014.
- 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