Southern Ireland (Irish: Deisceart Éireann) was the larger of the two parts of Ireland that were created when Ireland was partitioned under the Government of Ireland Act 1920. It comprised 26 of the 32 counties of Ireland or about five-sixths of the area of the island, whilst the remaining six counties in the northeast of the island formed Northern Ireland. Southern Ireland included County Donegal, despite it being the largest county in Ulster and the most northerly county in all of Ireland.
The Act of 1920, which came into force on 3 May 1921, was intended to create two self-governing territories within Ireland, each with its own parliament and governmental institutions, and both remaining within the United Kingdom of Great Britain and Ireland. It also contained provisions for co-operation between the two territories and for the eventual reunification of Ireland. However, in the 1921 elections for Southern Ireland's House of Commons, Sinn Féin candidates were returned unopposed in 124 of the 128 seats, and ignored the parliament, assembling instead as the Second Dáil. The "Parliament of Southern Ireland"—consisting of the four unionist members—met only once. Continuing unrest led to the Anglo-Irish Treaty and the Provisional Government which administered Southern Ireland from 16 January 1922 to 5 December 1922: effectively a transitional administration for the period between the ratifying of the Anglo-Irish Treaty and the establishment of the Irish Free State. Its legitimacy was disputed by the Anti-Treaty delegates to Dáil Éireann.
Southern Ireland, as a political entity, was superseded by the Irish Free State (which later became the independent state of Ireland) on 6 December 1922.