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 The Hon. 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 was elected to Westminster for the Ulster Unionist Party at the 1952 North Antrim by-election, succeeding his father. He stood down at the 1959 general election. At the 1958 Stormont elections, he was elected, again to represent North Antrim. In 1969, he briefly served as Minister of Education before becoming the Minister of Agriculture. In 1958, he was appointed High Sheriff of Antrim.
O'Neill joined the Alliance Party of Northern Ireland in 1972, and acted as its leader at the Darlington Conference. At the 1973 Northern Ireland Assembly election, he was unsuccessful in North Antrim. In 1982, he succeeded his father as the second Lord Rathcavan.