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.
Partition of Ireland
Soon after the end of the war Sinn Féin won the overwhelming majority of the Irish parliamentary seats in the UK general election of 1918, and in January 1919 the Sinn Féin members declared unilaterally an independent (all-island) Irish Republic. Unionists, however, won a majority of seats in four of the nine counties of Ulster and affirmed their continuing loyalty to the United Kingdom. Following the Paris Peace Conference, in September 1919 Lloyd George tasked the "Long Committee" to implement Britain's commitment to introduce Home Rule, which was based on the policy of Walter Long, and some findings of the Irish Convention. The result was to be two home-rule Irish jurisdictions, and in November 1920 the Government of Ireland Act 1920 was enacted. As a result of this, in April 1921 the island was partitioned into Southern and Northern Ireland.