Republic of Ireland–United Kingdom border
Originally intended as an internal boundary within the United Kingdom of Great Britain and Ireland, the border was created in 1921 under the United Kingdom Parliament's Government of Ireland Act 1920. Prior to this, a separatist Irish parliament had been established in Dublin, which did not recognise the Government of Ireland Act, and was actively engaged in the Irish War of Independence. The Act was intended to deliver Home Rule in Ireland, with separate parliaments for Southern Ireland (which included three of the nine counties of Ulster) and Northern Ireland. Six of the thirty-two counties of Ireland were assigned to Northern Ireland, and the rest of Ireland comprising 26 counties to Southern Ireland.
History of Northern Ireland
Northern Ireland is one of the four countries of the United Kingdom (although it is also described by official sources as a province or a region), situated in the northeast of the island of Ireland. It was created as a separate legal entity on 3 May 1921, under the Government of Ireland Act 1920. The new autonomous Northern Ireland was formed from six of the nine counties of Ulster: four counties with unionist majorities and two counties, Fermanagh and Tyrone, which had slight Irish nationalist majorities (of 53.6 and 54.6 per cent respectively in the 1918 election). The remaining three Ulster counties with larger nationalist majorities were not included. In large part unionists, at least in the northeast, supported its creation while nationalists were opposed.