The British Government, committed to implementing Home Rule, set up a cabinet committee under the chairmanship of southern unionist Walter Long.
The Long Committee recommended the establishment of two devolved administrations, dividing the island into two territories: Northern Ireland and Southern Ireland.
Today the former is still known as Northern Ireland and forms part of the United Kingdom, while the latter is now a sovereign state also named Ireland and sometimes called the Republic of Ireland.
The Act of 1920 was intended to create two self-governing territories within Ireland, with both remaining within the United Kingdom.
The Ulster Volunteer Force (UVF) imported 25,000 rifles and three million rounds of ammunition from the German Empire in the Larne gun-running of April 1914, and there were fears that passing the Home Rule Bill could start a full-scale civil war in Ulster.
The Curragh incident on 20 March 1914 had already led Westminster to believe that the British Army could not be trusted to carry out their orders in Ireland.
A year later on 6 December 1922, the Irish Free State became independent of the United Kingdom in accordance with the Treaty, which was given legislative effect in the United Kingdom by the Irish Free State (Agreement) Act 1922.