House of Commons of Southern Ireland was the lower house of the Irish parliament created by the Government of Ireland Act, passed in 1920, during the Irish War of Independence. The Act created two partitioned Irish states, Northern Ireland and Southern Ireland, each with their own two chamber assemblies, a House of Commons and a Senate.

In 1921, elections were held for the House of Commons of Southern Ireland. In reality, no contests occurred. All 128 MPs were returned unopposed - Sinn Féin won all 124 seats for geographic constituencies, whilsts Unionists won the four seats for graduates of Dublin University. The Irish Republic chose to regard that election as elections to the Second Dáil. The 124 Sinn Féin candidates elected, plus the Sinn Féin members elected to the House of Commons of Northern Ireland elected at the same time, assembed as the Second Dáil.

In June 1921, the House of Commons, together with the appointed Senate, formally assembled in the Royal College of Science , now Government Buildings, in Merrion St., for a state opening by the Lord Lieutenant of Ireland Viscount Fitzalan of Derwent.

In reality only four unionist MPs and a handful of appointed senators, turned up. Parliament was suspended.

The House of Commons of Southern Ireland came back into being later, as a result of the Anglo-Irish Treaty of 1921. The Treaty was submitted to two bodies, the Second Dáil, whose approval gave it legitimacy in the eyes of nationalist Ireland, and the House of Commons of Southern Ireland, which had legitimacy according to British constitutional theory, it being a creation of the King-in-Parliament. Both parliaments then chose their own governments, a republican administration under President of Dáil Éireann, Arthur Griffith and a Provisional Government under Michael Collins. The duality was represented by the meeting of Provisional Prime Minister Michael Collins and the Lord Lieutenant, Viscount Fitzalan of Derwent. According to Irish republican history, Collins was 'accepting the surrender of Dublin Castle', where the meeting occurred. In contrast, in British political theory, Fitzalan met Collins to kiss hands, (ie to formally install him as a Minister of the Crown).

The House of Commons of Southern Ireland, having chosen Collins' government, was then dissolved and replaced by a new united parliament, called alternatively the Constituent Assembly (i.e. a parliament with the authority to create a constitution), the Third Dáil or the Provisional Parliament, depending on whose political theory one accepts. The unique status of this parliament was shown in the fact that it was presided over by the Ceann Comhairle (pronounced, keaun corrle), the title given to the speaker in Dáil Éireann yet received messages from the Lord Lieutenant. Some anti-treaty deputies challenged the exact status of the assembly; whether it was a Dáil or a successor to the House of Commons of Southern Ireland. The deaths of Arthur Griffith and Michael Collins led to the merging of their offices under the united leadership of William T. Cosgrave. With the coming into being of the Irish Free State and its constitution in December 1922, all previous administrations and parliaments, both nationalist and created by British law, ceased to exist.