The colonial conquests rendered Irish Catholics non-citizens: Catholics were not formally emancipated in the United Kingdom until 1829.
The rise of Irish nationalism and unrest in the nineteenth century led to failed home rule efforts, the Easter Rising of 1916, and unrest that continued into 1923.
The British partition of Ireland in 1920 divided that part of historic Ulster that had been most successfully incorporated into the United Kingdom (what became Northern Ireland) from the rest of Ireland,
which became independent as the Irish Free State (later the Republic of Ireland).
Fearful that they would one day be betrayed by Britain and that Irish nationalists were intent on forcibly reunifying Ireland,
Ulster Unionists ran a systematically discriminatory regime from 1921 until the British government suspended the Northern Ireland parliament in 1972.
Even after World War Two,
Catholics faced discrimination, especially in obtaining government employment and access to housing,
but also in the private sector.