The 1921 Anglo-Irish Treaty (Irish: An Conradh Angla-ireannach), 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 representatives of the Irish Republic 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 "community of nations known as the British Empire", a status "the same as that of the Dominion of Canada". 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 the Parliament of Northern Ireland 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 by representatives of the Irish Republic including Michael Collins and Arthur Griffith. The Irish representatives had plenipotentiary status (negotiators empowered to sign a treaty without reference back to their superiors) acting on behalf of the Irish Republic, though the British government declined to recognise that status. As required by its terms, the agreement was approved by "a meeting" of the members elected to sit in the House of Commons of Southern Ireland and [separately] by the British Parliament. In reality, Dil ireann (the legislative assembly for the de facto Irish Republic) first debated then approved the treaty; members then went ahead with the "meeting". Though the treaty was narrowly approved, the split led to the Irish Civil War, which was won by the pro-treaty side.

The Irish Free State as contemplated by the treaty came into existence when its constitution became law on 6 December 1922 by a royal proclamation.

Dáil Éireann ( (listen) doyl AIR-ən, Irish: [ˌd̪ˠaːlʲ ˈeːɾʲən̪ˠ]; lit. 'Assembly of Ireland') is the lower house, and principal chamber, of the Oireachtas (Irish legislature), which also includes the President of Ireland and Seanad Éireann (the upper house). It consists of 160 members, each known as a Teachta Dála (plural Teachtaí Dála, commonly abbreviated as TDs). TDs represent 39 constituencies and are directly elected for terms not exceeding five years, on the system of proportional representation by means of the single transferable vote (PR-STV). Its powers are similar to those of lower houses under many other bicameral parliamentary systems and it is by far the dominant branch of the Oireachtas. Subject to the limits imposed by the Constitution of Ireland, it has power to pass any law it wishes, and to nominate and remove the Taoiseach (head of government). Since 1922, it has met in Leinster House in Dublin.

The Dáil took its current form when the 1937 Constitution was adopted, but it maintains continuity with the First Dáil established in 1919.