A United Ireland is the common demand of Irish nationalists, envisaging that the island of Ireland be united as a political entity. However, nationalists have suggested many different models, including federalism, joint sovereignty and joint authority , as well as a unitary state.

Despite several nationalists and republicans expressing a wish for the "Re-Unification of Ireland" - which implies a former time of political unity - the island of Ireland has never existed as a sovereign, independent political state. Various off-shoots of the Ui Néill did become High Kings of Ireland but this title merely denoted the most powerful king in Ireland, not the ruler of the entire nation.

Two indigenious Irish dynasties, the O'Brians and the O'Connors, came close to creating an Ireland united under a High King in the 11th and 12th centuries, but failed.

The next significant moment occurred in 1642 when the Irish Confederate Catholics assembled at Kilkenny and held an all-Ireland assembly. The Confederates did rule much of Ireland up to 1649, but never the entire island and were riven by dissent and civil war in later years.

In 1919 Dail Eireann declared itself the Government of the Republic of Ireland and independent of the British Empire. Its claims over the entire island were never accepted by the Unionists of Northern Ireland. In 1922 the Irish Free State became the name of the state covering the twenty-six counties in the south, while Northern Ireland remained within the UK.

The Free State and its successor, the Republic of Ireland (declared in 1949) both claimed jurisdiction over the six counties contained in Northern Ireland, but never seriously attempted to integrate them into the Free State or the Republic. In 1999 the Republic gave up all claim to the territory of Northern Ireland.

The leading political party in the Republic of Ireland, Fianna Fáil, as well as Sinn Féin, the SDLP and other parties have often made a united Ireland a central part of their political message. However, the Unionist community – composed primarily of Protestants in the six counties that form Northern Ireland – oppose unification. All of the island's political parties (except for tiny fringe groups with no electoral representation) have accepted the principle of consent which states that Northern Ireland's constitutional status cannot change without majority support in Northern Ireland.

The Protestants of the North (who generally refer to Northern Ireland as 'Ulster') argue they have a distinct national identity that would be destroyed in a united Ireland. They cite the decline of the Protestant population of the 26 counties of the Republic of Ireland. Most nationalists retort that that decline is for reasons other than persecution and all would cite a very different political context for a unified island today than in 1921.

A possible referendum on a united Ireland was included as part of the terms of the Good Friday Agreement.