Dáil Éireann ratifies the Anglo-Irish Treaty by a 64–57 vote.[citation needed]

Dáil Éireann is the lower house and principal chamber of the Oireachtas, which also includes the president of Ireland and a senate called Seanad Éireann. It consists of 174 members, each known as a Teachta Dála. TDs represent 43 constituencies and are directly elected for terms not exceeding five years, on the system of proportional representation using 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 the power to pass any law it wishes, and to nominate and remove the Taoiseach. Since 1922, it has met in Leinster House in Dublin.

The 1921 Anglo-Irish Treaty, commonly known in Ireland 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 the government 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 was exercised by the Parliament of Northern Ireland.