From Wikipedia: "The Anglo-Irish Treaty (Irish: An Conradh Angla-Éireannach), officially called the Articles of Agreement for a Treaty Between Great Britain and Ireland, was a treaty between the Government of the United Kingdom of Great Britain and Ireland and representatives of the secessionist Irish Republic that concluded the Irish War of Independence. It established the Irish Free State as a self-governing dominion within the British Commonwealth of Nations (the first use of this term, rather than British Empire, in an imperial statute) and 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.
The treaty was signed in London on 6 December 1921 by representatives of the British government (which included Prime Minister David Lloyd George, who was head of the British delegates) and envoys of the Irish Republic, including Michael Collins and Arthur Griffith, who claimed plenipotentiary status (i.e. negotiators empowered to sign a treaty without reference back to their superiors). In accordance with its terms, the Treaty was required to be, and was, ratified by the members elected to sit in the House of Commons of Southern Ireland and the British Parliament. Dáil Éireann for the de facto Irish Republic also ratified the Treaty. Though the treaty was narrowly ratified, the split led to the Irish Civil War, which was ultimately won by the pro-treaty side.
The Irish Free State created by the Treaty came into force on 6 December 1922 by royal proclamation after its constitution had been enacted by the Constituent Assembly (the Third Dáil) and the British parliament."