Dáil Éireann votes to approve the Anglo-Irish Treaty on January 7, 1922, following a debate through late December 1921 and into January 1922. The vote is 64 in favour, 57 against, with the Ceann Comhairle and three others not voting. The Sinn Féin party splits into opposing sides in the aftermath of the Treaty vote, which leads to the Irish Civil War from June 1922 until May 1923. The treaty is signed in London on December 6, 1921.

Two elections take place in Ireland in 1921, as a result of the Government of Ireland Act 1920 to establish the House of Commons of Northern Ireland and the House of Commons of Southern Ireland. The election is used by the Irish Republic as the basis of membership of the Second Dáil. The general election to the Northern Ireland House of Commons occurs on May 24, 1921. Of 52 seats, forty are won by unionists, six by moderate Irish nationalists, and six by Sinn Féin. No actual polling takes place in the Southern Ireland constituencies, as all 128 candidates are returned unopposed. Given the backdrop of the increasingly violent War of Independence, any candidates opposed to Sinn Féin, and their supporters can expect to be harassed by the Irish Republican Army (IRA). Supporters of the Irish Labour Party stand aside to allow the constitutional situation to run its course. Of these 128, 124 are won by Sinn Féin, and four by independent unionists representing the University of Dublin constituency. Only the Sinn Féin candidates recognise the Second Dáil and five of these have been elected in two constituencies, one in each part of Ireland, so the total number of members who assemble in the Second Dáil is 125.

During the Second Dáil, the government of the Irish Republic and the British government of David Lloyd George agree to hold peace negotiations. On September 14, 1921, the Dáil ratifies the appointment of Arthur Griffith, Michael Collins, Robert Barton, Eamonn Duggan, and George Gavan Duffy as envoys plenipotentiary for the peace conference in England. These envoys eventually sign the Anglo-Irish Treaty on December 6. After almost a month of acrimonious debate the treaty is formally ratified by Dáil Éireann on January 7, 1922.

To satisfy the requirements of the British constitution, the treaty also has to be ratified by the House of Commons of Southern Ireland. Thus, Irish nationalists end their boycott of the home rule parliament to attend the southern House of Commons as MPs. This they do alongside the four Unionist MPs who had refused to recognise the Dáil. In this way the treaty is ratified a second time in Dublin, this time unanimously as the anti-Treaty TDs refuse to attend.

Under the terms of the Anglo-Irish Treaty a provisional parliament, considered by nationalists to be the Third Dáil, is elected in the 1922 general election on June 16. Collins and Éamon de Valera agree a pact between the pro- and anti-Treaty wings of Sinn Féin, and this pact and the elections are endorsed by the Second Dáil. The new assembly is recognised both by nationalists and the British Government and so replaces both the Parliament of Southern Ireland and the Second Dáil with a single body. The anti-Treaty groups of IRA members, TDs, and their supporters are still bitterly opposed the settlement, despite the election result, and this leads to the Irish Civil War.

(Pictured: Second All-Ireland Dáil Éireann, elected in 1921)