The Government of Ireland Act 1920—the Fourth Home Rule Act, on paper the answer to four decades of Irish constitutional argument—took effect on 3 May 1921. It split the island into two self-governing territories within the United Kingdom. Northern Ireland: six counties, mostly Ulster, mostly Protestant, mostly industrial. Southern Ireland: the other 26 counties, about five-sixths of the island, mostly Catholic, mostly agricultural. Each would have its own parliament, government, supreme court. A Council of Ireland would coordinate between them, supposedly leading to eventual reunification. The architecture was elaborate. The intention was conciliation. The result was that one of the two new entities functioned. The other—Southern Ireland—was treated by its own electorate as a fiction.

In May 1921 the first general election to the House of Commons of Southern Ireland was held simultaneously with the Northern Ireland election. In Southern Ireland, every Sinn Féin candidate was returned unopposed. There was, in practical terms, no polling. Sinn Féin had decided that any seat won under the 1920 Act was a seat in the Republic's Dáil, not in a Westminster-designed colonial parliament. So 124 of 128 seats went to Sinn Féin members who promptly walked across town to constitute themselves the Second Dáil Éireann. The Royal College of Science meeting on 28 June lasted just long enough to confirm that the parliament could not function. The Senate met twice, in similar conditions. Lord FitzAlan of Derwent, the Lord Lieutenant, presided over the formal absence of a state.

While Southern Ireland's parliament refused to meet, the Anglo-Irish War continued. On 6 December 1921, after months of secret negotiation, the Anglo-Irish Treaty was signed in London. It created the Irish Free State, a self-governing dominion with the same constitutional status as Canada. The Treaty required ratification. The mechanism chosen was telling: not the House of Commons of Southern Ireland as a sitting parliament, but a meeting of 'the members elected to sit' in it—a phrase carefully chosen to allow Sinn Féin members to attend without acknowledging the 1920 Act. On 14 January 1922, Arthur Griffith, as Chairman of the Irish Delegation of Plenipotentiaries, convened the meeting. It approved the Treaty. The Provisional Government was constituted, and two days later Michael Collins took charge of Dublin Castle as Chairman.

From 16 January 1922 to 5 December 1922, the Provisional Government—not Southern Ireland's nominal institutions—governed the 26 counties. It was a transitional regime, established by the Treaty rather than the Government of Ireland Act, designed to bridge the gap until the Free State's constitution took effect. Anti-Treaty Sinn Féin disputed its legitimacy. The disagreement became the Irish Civil War. On 27 May 1922, Lord FitzAlan formally dissolved the Parliament of Southern Ireland and proclaimed a new 'Provisional Parliament' under the Irish Free State (Agreement) Act 1922. The British constitutional framework was being dismantled even as the Irish Civil War's first shots were being fired. The Republic that Sinn Féin had refused to renounce was being replaced by a Free State that some of them refused to accept.

Southern Ireland ceased to exist on 6 December 1922. The Irish Free State took its place that day, with a new constitution, a new parliament called the Oireachtas, and a new official identity that the Republic of 1937 would eventually inherit. In all of Irish constitutional history, Southern Ireland is the briefest of resting places—a name that appears on a few months of letterhead and in the formal title of an Anglo-Irish Treaty. Its parliament met once with four members. Its senate met twice. Its courts heard a small number of cases before being abolished. It is the closest thing modern European history has to a country that everyone agreed not to use. The line drawn by the 1920 Act still exists, though—in the border between the Republic and Northern Ireland, the only land border between an EU member state and the United Kingdom, still negotiating a hundred years later what it means.