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.
Monarchy of the United Kingdom
In 1800, as a result of the Irish Rebellion of 1798, the Act of Union merged the kingdom of Great Britain and the kingdom of Ireland into the United Kingdom of Great Britain and Ireland. The whole island of Ireland continued to be a part of the United Kingdom until 1922, when what is now the Republic of Ireland won independence as the Irish Free State, a separate Dominion within the Commonwealth. The Irish Free State was renamed Éire (or "Ireland") in 1937, and in 1949 declared itself a republic, left the Commonwealth and severed all ties with the monarchy. Northern Ireland remained within the Union. In 1927, the United Kingdom changed its name to the United Kingdom of Great Britain and Northern Ireland, while the monarch's style for the next twenty years became "of Great Britain, Ireland and the British Dominions beyond the Seas, King, Defender of the Faith, Emperor of India".