Background and education
O'Neill was the third son of Edward O'Neill, 2nd Baron O'Neill, and the uncle of Terence O'Neill, Prime Minister of Northern Ireland. Educated at Eton College and New College, Oxford, Hugh O'Neill was subsequently called to the Bar at Inner Temple. He served as a Major in the British Army.
O'Neill was also elected to represent Antrim in the Northern Ireland House of Commons in 1921 and served as its first Speaker, before standing down from his seat in 1929. The latter year he was created a Baronet, of Cleggan in the County of Antrim. In 1934, he was appointed High Sheriff of Antrim.
From 1933 to 1939, O'Neill was the Chairman of the 1922 Committee. He sat on the Privy Council of Ireland, and was the sole surviving member of that body immediately prior to his death in 1982. He was also a member of its northern relation, the Privy Council of Northern Ireland and the Privy Council of the United Kingdom. From 1939 to 1940, he was the Parliamentary Under-Secretary of State for India and Burma, and was the Lord Lieutenant of Antrim from 1949 to 1959.
O'Neill retired from the Westminster Parliament in 1952, having become the Father of the House the previous year, and was raised to the peerage as Baron Rathcavan, of The Braid in the County of Antrim, in 1953.
Lord Rathcavan died in 1982 at the age of 99 and was succeeded by his eldest surviving son, Phelim.
- Northern Ireland Parliamentary Elections Results: Biographies
- Leigh Rayment's Peerage Pages [self-published source][better source needed]
- Leigh Rayment's Historical List of MPs
- Hansard 1803–2005: contributions in Parliament by The Lord Rathcavan
|Parliament of the United Kingdom|
| Member of Parliament for Mid Antrim
|New constituency|| Member of Parliament for Antrim
With: Charles Craig 1922–1929
Sir Joseph McConnell 1929–1942
John Dermot Campbell 1943–1945
Samuel Gillmor Haughton 1945–1950
|New constituency|| Member of Parliament for North Antrim
| Father of the House
|New title|| Speaker of the Northern Ireland House of Commons
| Under-Secretary of State for India and Burma
Duke of Devonshire
James Graham Leslie
| Lord Lieutenant of Antrim
Sir Richard Dobbs
|Peerage of the United Kingdom|
|New title|| Baron Rathcavan
Phelim Robert Hugh O'Neill