The Anglo-Irish Treaty (Irish: An Conradh Angla-Eireannach), officially 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 provided for the establishment within a year of an Irish Free State 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.
The agreement 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). As required by its terms, the agreement was ratified by the members elected to sit in the House of Commons of Southern Ireland and the British Parliament, and so became a treaty. Dail Eireann 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 Dail) and the British parliament.
Among the Treaty's main clauses were that:
British forces would withdraw from most of Ireland.
Ireland was to become a self-governing dominion of the British Empire, a status shared by Australia, Canada, Newfoundland, New Zealand and the Union of South Africa.
As with the other dominions, the British monarch would be the head of state of the Irish Free State (Saorstat Eireann) and would be represented by a Governor General (See Representative of the Crown).
Members of the new free state's parliament would be required to take an Oath of Allegiance to the Irish Free State. A secondary part of the Oath was to "be faithful to His Majesty King George V, his heirs and successors by law, in virtue of the common citizenship".
Northern Ireland (which had been created earlier by the Government of Ireland Act) would have the option of withdrawing from the Irish Free State within one month of the Treaty coming into effect.
If Northern Ireland chose to withdraw, a Boundary Commission would be constituted to draw the boundary between the Irish Free State and Northern Ireland.
Britain, for its own security, would continue to control a limited number of ports, known as the Treaty Ports, for the Royal Navy.
The Irish Free State would assume responsibility for a proportionate part of the United Kingdom's debt, as it stood on the date of signature.
The Treaty would have superior status in Irish law, i.e., in the event of a conflict between it and the new 1922 Constitution of the Irish Free State, the treaty would take precedence.