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.
Dissident Irish Republican campaign
Since the 1169 invasion of Ireland by Norman knights at the request of ousted King of Leinster Dermot MacMurrough, Ireland has, in part or in whole, been under English, and later British, administration. Rebellions against rule from Great Britain were unsuccessful until 1919–1921's Anglo-Irish War, when the original Irish Republican Army (IRA) succeeded in removing 26 of Ireland's 32 counties from the United Kingdom of Great Britain and Ireland as the Irish Free State. Although previously, the offer of Irish Home rule was agreed in the third Home Rule Bill, implementation was suspended by violent opposition in Ulster and the forming of the UVF, and later by the outbreak of the First World War. Following the partition of Ireland by the Crown, the remaining six counties, located in the province of Ulster, became Northern Ireland and remained a part of the renamed United Kingdom of Great Britain and Northern Ireland.