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
In May 1949 the Taoiseach John A. Costello introduced a motion in the Dáil strongly against the terms of the UK's "Ireland Act 1949" that confirmed partition for as long as a majority of the electorate in Northern Ireland wanted it, styled in Dublin as the "Unionist Veto". This was a change from his position supporting the Boundary Commission back in 1925, when he was a legal adviser to the Irish government. A possible cause was that his coalition government was supported by the strongly republican Clann na Poblachta. From this point on all the political parties in the Republic were formally in favour of ending partition regardless of the opinion of the electorate in Northern Ireland.