South Derry was a parliament constituency in Ireland which returned one Member of Parliament to the House of Commons of the United Kingdom on the electoral system of first past the post from 1885 to 1922.
The constituency was a majority unionist area. All three candidates at the 1918 general election were Catholics.
Prior to 1885, County Londonderry returned two MPs to the House of Commons of the United Kingdom sitting at the Palace of Westminster. Under the Redistribution of Seats Act 1885, the parliamentary county was divided into the divisions of North Derry and South Derry. The parliamentary borough of Coleraine lost its separate representation.
South Derry was defined as:
It was not affected by the Redistribution of Seats (Ireland) Act 1918. Sinn Féin contested the 1918 general election on an abstentionist platform that instead of taking up any seats at Westminster, they would establish a revolutionary assembly in Dublin. All MPs elected to Irish seats were invited to participate in the First Dáil convened in January 1919, but no members outside of Sinn Féin did so.
The Government of Ireland Act 1920 established the Parliament of Northern Ireland, which came into operation in 1921. The representation of Northern Ireland at Westminster was reduced from 30 MPs to 13 MPs, taking effect at the 1922 United Kingdom general election. At Westminster, Londonderry City, North Derry and South Derry were combined to form the single-seat county constituency of Londonderry. A five-seat constituency of Londonderry was created for the House of Commons of Northern Ireland, which formed the basis in republican theory for representation in the Second Dáil.
Gordon is appointed a judge, prompting a by-election.
Henry is appointed Lord Chief Justice of Northern Ireland, prompting a by-election.
Chichester dies, causing a by-election.