Homosexual Offences (Northern Ireland) Order 1982
The Homosexual Offences (Northern Ireland) Order 1982, No. 1536 (N.I. 19), was an Order in Council which decriminalized homosexual acts between consenting adults in Northern Ireland, bringing Northern Ireland into line with the rest of the United Kingdom, with an age of consent of 21.
The Order was adopted as a result of a European Court of Human Rights case, Dudgeon v United Kingdom (1981), which ruled that Northern Ireland's criminalisation of homosexual acts between consenting adults was a violation of Article 8 of the European Convention on Human Rights. At that time there was no devolved assembly in Nortern Ireland, so legislation was enacted by the British Government by Order in Council.
http://www.legislation.gov.uk/nisi/1982/1536#reference-c546654 Text of the Order, incorporating subsequent revisions changing the age of consent to 17.