Terence O'Neill, Baron O'Neill of the Maine, Anglo-Irish captain and politician, 4th Prime Minister of Northern Ireland (died 1990)

Terence Marne O'Neill, Baron O'Neill of the Maine, PC (NI), was the fourth Prime Minister of Northern Ireland and leader (1963–1969) of the Ulster Unionist Party (UUP). A moderate unionist who sought to reconcile sectarian divisions in Northern Ireland society and met with his counterpart in the Irish Republic, he was a member of the Parliament of Northern Ireland for the Bannside constituency from 1946 until his resignation in January 1970.

The prime minister of Northern Ireland was the head of the Government of Northern Ireland between 1921 and 1972. No such office was provided for in the Government of Ireland Act 1920; however, the Lord Lieutenant of Ireland, as with governors-general in other Westminster systems such as in Canada, chose to appoint someone to head the executive even though no such post existed in statute law. The office-holder assumed the title prime minister to draw parallels with the prime minister of the United Kingdom. On the advice of the new prime minister, the lord lieutenant then created the Department of the Prime Minister. The office of Prime Minister of Northern Ireland was suspended in 1972 and then abolished in 1973, along with the contemporary government, when direct rule of Northern Ireland was transferred to London.