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.
Irish Civil War
The fact that the Irish Civil War was fought between Irish Nationalist factions meant that the sporadic conflict in Northern Ireland ended. Collins and Sir James Craig signed an agreement to end it on 30 March 1922, but, despite this, Collins covertly supplied arms to the Northern IRA until a week before his death in August 1922. Because of the Irish Civil War, Northern Ireland was able to consolidate its existence and the partition of Ireland was confirmed for the foreseeable future. The continuing war also confirmed the northern Unionists' existing prejudices against the ethos of all shades of nationalism. This might have led to open hostilities between North and South had the Irish Civil War not broken out. Indeed, the Ulster Special Constabulary (the "B-Specials") that had been established in 1920 (on the foundation of Northern Ireland) was expanded in 1922 rather than being demobilised.