Unionism, in Ireland, is a belief in the desirability of a full constitutional and institutional relationship between Ireland and Great Britain based on the terms and order of government of the Act of Union 1800 which had merged both countries in 1801 to form the United Kingdom of Great Britain and Ireland (the successor entities being the United Kingdom of Great Britain and Northern Ireland and the Irish Free State). The term owes its origins to the campaigns by opponents of Irish home rule in the late 19th and early 20th centuries to prevent the creation of an all-Ireland home rule parliament within the United Kingdom. Because of their desire to maintain the Act of Union as created in 1800, without any system of devolution, they came to be known as Unionists.

Some believe that the unionist opposition to home rule was not simply based on a desire for a different structure for governance, but reflected a fundamental difference in perspective, beliefs, definition and culture between Irish Nationalists and Unionists. Whereas Nationalists were predominantly, but not exclusively, Roman Catholic, Unionists were predominantly, but not universally, Protestant. Almost all were descendants of English and Scottish settlers who arrived in the province of Ulster, especially from the Plantation of Ulster, in the early 17th century, onwards.