Phelim Robert Hugh O'Neill, 2nd Baron Rathcavan, PC (NI) (2 November 1909 – 20 December 1994), was a politician in Northern Ireland and a hereditary peer in the British House of Lords. The son of Hugh O'Neill, a unionist politician, Phelim studied at Eton College before joining the Royal Artillery. He became a major during World War II. O'Neill joined the Alliance Party of Northern Ireland in 1972, and acted as its leader at the . At the 1973 Northern Ireland Assembly election, he was unsuccessful in North Antrim. In 1982, he succeeded his father as the second Baron Rathcavan.