O'Neill, Phelim Robert Hugh (1909–94), 2nd Baron Rathcavan , politician, was born 2 November 1909 at The Braid, Ballymoney, Co. Antrim, eldest among three sons of Robert William Hugh O'Neill (qv) and Sylvia Irene O'Neill (née Sandeman) from Royston, Hertfordshire, England. His father was the first speaker of the Northern Ireland parliament, and the family had a long tradition in unionist politics; his cousin Terence O'Neill (qv) became NI prime minister 1963–9. His younger brother Con O'Neill (qv) was a senior diplomat in the British foreign service and one of the chief negotiators of Britain's entry into the EEC. He was educated at Eton, and always wore an old Etonian tie. With his family so entrenched in political life it was no surprise when he became unionist MP for Antrim North at Westminster in 1952, winning the by-election of October called on the resignation of his father. He described his politics as ‘left-wing conservative’ and for much of his political life he embraced such contradictions. An individualist, who was also labelled eccentric, he often found it difficult to adhere to the strictures of party political life.
On his father's retirement from the NI parliament in 1958, he won the seat, again for Antrim North, by a comfortable margin. In Stormont he became, if anything, more irreverent and it was not unusual for him to treat ministers rather scornfully. His speeches in the house were often imbued with his characteristic entertaining wit, but were seldom frivolous or pointless. He was regarded as a voice of reason in an atmosphere that was seldom conducive to bipartisanship. His ecumenical spirit embroiled him in controversy: he was expelled from the Orange order when he attended a catholic service in 1968 as part of his public duty. A strong advocate of reform in Northern Ireland, he supported his cousin Terence and was rewarded with a promotion to the cabinet in 1969 as minister of education. This was soon followed by a stint as agriculture minister (1969–71).
Under the premiership of Brian Faulkner (qv), O'Neill's position within the cabinet was not secure and he was eventually dropped. By then he had tired of sectarian politics and was among a trio of MPs to form the first Alliance party grouping in Stormont under the chairmanship of Oliver Napier, stating that he did ‘not feel the Unionist party is really capable of mobilising the forces of sanity here!’ (Times, 19 Feb. 1972). He doubted whether the new grouping would achieve much – ‘we do not have the two essentials for elections here: a sash and a drum or a parish priest’ (Times, 20 June 1973) – but felt that he had no choice but to try to make it work. He did not get an opportunity, failing to take a seat in the 1973 assembly elections, when the party could only secure eight out of seventy-eight seats. This defeat marked the end of his political career. He succeeded to the title of Lord Rathcavan on his father's death in 1982. In later life he divided his time between his residence at Aghadowey, Co. Londonderry, and Killala, Co. Mayo. He died 12 December 1994 in Mayo county hospital.
He married (12 February 1934) Clare Desirée, daughter of Detmar Blow of 3 Carlos Place, London. They divorced in 1953, but had a daughter and a son (Hugh Detmar Torrens O'Neill who served as chairman of the NI tourist board and Northern Ireland Airports). In 1953, he married Bridget Doreen, youngest daughter of Maj. the Hon. Richard Coke of Norfolk, England, with whom he had two daughters.