United Ireland, also referred to as Irish reunification or a New Ireland, is the proposition that all of the island of Ireland should be a single sovereign state. At present, the island is divided politically: the sovereign state of Ireland has jurisdiction over the majority of Ireland, while Northern Ireland, which lies entirely within the Irish province of Ulster, is part of the United Kingdom. Achieving a united Ireland is a central tenet of Irish nationalism and Republicanism, particularly of both mainstream and dissident republican political and paramilitary organisations. Unionists support Northern Ireland remaining part of the United Kingdom and oppose Irish unification.

Northern Ireland is a part of the United Kingdom in the north-east of the island of Ireland that is variously described as a country, province or region. Northern Ireland shares an open border to the south and west with the Republic of Ireland. At the 2021 census, its population was 1,903,175, making up around 3% of the UK's population and 27% of the population on the island of Ireland. The Northern Ireland Assembly, established by the Northern Ireland Act 1998, holds responsibility for a range of devolved policy matters, while other areas are reserved for the UK Government. The government of Northern Ireland cooperates with the government of Ireland in several areas under the terms of the Belfast Agreement. The Republic of Ireland also has a consultative role on non-devolved governmental matters through the British–Irish Governmental Conference (BIIG).