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.
United Kingdom of Great Britain and Ireland
Growing desire for Irish self-governance led to the Irish War of Independence, which resulted in most of Ireland seceding from the Union and forming the Irish Free State in 1922. Northern Ireland remained part of the Union, and the state was renamed to the current "United Kingdom of Great Britain and Northern Ireland" in 1927. The modern-day United Kingdom is the same country as the one from this period—a direct continuation of what remained after the secession—not an entirely new successor state.