The Anglo-Irish Treaty , commonly known as The Treaty and officially the Articles of Agreement for a Treaty Between Great Britain and Ireland, was an agreement between the government of the United Kingdom of Great Britain and Ireland and Irish representatives that concluded the Irish War of Independence. It provided for the establishment of the Irish Free State within a year as a self-governing dominion within the British Commonwealth of Nations (the first use by the UK government of this term, rather than "British Empire", in an official document). It also provided Northern Ireland, which had been created by the Government of Ireland Act 1920, an option to opt out of the Irish Free State, which it exercised.