A politically United Ireland is the common goal of Irish republicans and Irish nationalists, envisaging that the island of Ireland (currently divided into the Republic of Ireland, a sovereign state, and Northern Ireland, a part of the United Kingdom) be united as a single and independent political entity. Many different models for unification have been suggested including federalism and joint sovereignty, as well as a unitary state.

Since the English Tudor re-conquest of Ireland in the mid-16th century, a series of measures, both administrative and militaristic, were brought in to deal with Irish resistance to the English administration based at Dublin Castle. These were met with centuries of opposition and political violence. In 1920 the island was partitioned into what would become the Republic of Ireland, where opinion was almost uniformly in favour of independence from the United Kingdom of Great Britain and Ireland, and Northern Ireland, where the population was divided 60:40 in favour of remaining within the United Kingdom.

During the 20th century, political violence continued in Northern Ireland, but was virtually unheard of in the Republic. However, in the Republic reunification of the island remains an important political ambition, though one pursued through constitutional means. In Northern Ireland, reunification is a goal of Nationalist political parties and of Irish republican paramilitary groups. Opposition to a united Ireland is, conversely, a policy of Unionist political parties and of loyalist paramilitary groups.