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Armagh or County Armagh is a former county constituency in the House of Commons of the United Kingdom. It was a two-member constituency in Ireland from 1801 to 1885 and a single-member constituency in Northern Ireland from 1922 to 1950. It was replaced in boundary changes in 1983.

County Armagh had been represented by two seats in the Irish House of Commons. From 1 January 1801, when the Acts of Union 1800 came into effect, it was represented by a county constituency with two MPs in the United Kingdom House of Commons. It consisted of County Armagh, except the parts in the parliamentary borough constituencies of Armagh City and Newry. The borough of Charlemont was disfranchised in 1801. The First Parliament of the United Kingdom in 1801 was composed all members of the parliament of Great Britain and members continuing from areas which were enfranchised in Ireland.

Catholics were excluded from taking seats until the enactment of the Roman Catholic Relief Act 1829. Under the Parliamentary Elections (Ireland) Act 1829, the traditional county 40 shilling freehold landowning qualification was changed to a £10 qualification (which was an increase to five times the previous level). Before the Representation of the People Act 1884, there was a restrictive property-based franchise. It was not until the householder franchise was introduced for county elections, in the electoral reforms which took effect in 1885, that most (but not all) adult males became voters. In these circumstances, most members of parliament came from a limited number of Protestant aristocratic and gentry families. There were few contested elections.

In the first half-century or so after the union, the constituency was fairly evenly balanced between Whig/Liberal and Tory/Conservative parties. Thereafter the area became more Conservative.

In 1885, under the Redistribution of Seats Act 1885, it was split into Mid Armagh, North Armagh and South Armagh.^[1]

In 1922, following the establishment of the Parliament of Northern Ireland, the number of seats in Northern Ireland at Westminster was cut from 30 to 13 under the Government of Ireland Act 1920.^[2] The single-seat constituency of Armagh consisted of the entirety of County Armagh. From its inception, Armagh had a unionist majority, though by the 1970s the nationalist vote was in the mid-30s%. In 1951, it was one of the last four seats to be uncontested in a UK general election, and in 1954 it saw the last uncontested by-election in the UK. In 1974 the Ulster Unionist Party repudiated the Sunningdale Agreement and so did not reselect the pro-Sunningdale MP, John Maginnis. Instead, they ran Harold McCusker, who held the seat until 1983. He was then elected for Upper Bann, which contained part of Armagh.

In 1983 most of the constituency became part of the Newry and Armagh constituency, with part going to Upper Bann.^[3]

In two-member elections, the bloc voting system was used. Voters could cast a vote for one or two candidates, as they chose. The two candidates with the largest number of votes were elected.

In by-elections to fill a single seat, the first past the post system applied.

After 1832, when registration of voters was introduced, a turnout figure is given for contested elections. In two-member elections, when the exact number of participating voters is unknown, this is calculated by dividing the number of votes by two. To the extent that voters did not use both their votes this will be an underestimate of turnout. If the electorate figure is unknown the last known electorate figure is used to provide an estimate of turnout.

Where a party had more than one candidate in one or both of a pair of successive elections change is calculated for each individual candidate, otherwise change is based on the party vote.