Irish republicanism is the political movement for the unity and independence of Ireland under a republic. Irish republicans view British rule in any part of Ireland as inherently illegitimate. An ideology since the 17th century, various methods have been employed to achieve the republic, including rebellions and paramilitary campaigns. Although the makeup of republicanism has been multidenominational, its relation to catholicism increasingly became central.

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.