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.

O'Neill was elected to Westminster for the Ulster Unionist Party at the 1952 North Antrim by-election, succeeding his father.

[1] 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.