Armagh was a county constituency of the Parliament of Northern Ireland from 1921 to 1929.

It returned four MPs, using proportional representation by means of the single transferable vote.

The House of Commons (Method of Voting and Redistribution of Seats) Act (Northern Ireland) 1929 divided the constituency into four constituencies elected under first past the post: Central, Mid, North and South Armagh constituencies.

[1] In May 1921, Dáil Éireann, the parliament of the self-declared Irish Republic run by Sinn Féin, passed a resolution declaring that elections to the House of Commons of Northern Ireland and the House of Commons of Southern Ireland would be used as the election for the Second Dáil.

[3] Armagh had a slight Unionist majority, but this was fairly evenly balanced with a Nationalist minority.