Under the British Government of Ireland Act, Ireland was partitioned into two self-governing territories, Northern and Southern Ireland.

The Government of Ireland Act 1920 was an act of the Parliament of the United Kingdom. The Act's long title was "An Act to provide for the better government of Ireland"; it is also known as the Fourth Home Rule Bill or (inaccurately) as the Fourth Home Rule Act and informally known as the Partition Act. The Act was intended to partition Ireland into two self-governing polities: the six north-eastern counties were to form "Northern Ireland", while the larger part of the country was to form "Southern Ireland". Both territories were to remain part of the United Kingdom of Great Britain and Ireland and provision was made for their future reunification through a Council of Ireland. The Act was passed by the British Parliament in November 1920, received royal assent in December and came into force on 3 May 1921.

The partition of Ireland was the process by which the Government of the United Kingdom of Great Britain and Ireland (UK) divided Ireland into two self-governing polities: Northern Ireland and Southern Ireland. It was enacted on 3 May 1921 under the Government of Ireland Act 1920. The Act intended both territories to remain within the United Kingdom and contained provisions for their eventual reunification. The smaller Northern Ireland territory was created with a devolved government and remained part of the UK. Although the larger Southern Ireland was also created, its administration was not recognised by most of its citizens, who instead recognised the self-declared 32-county Irish Republic.

Northern Ireland is a part of the United Kingdom in the north-east of the island of Ireland. It has been variously described as a country, province or region. Northern Ireland shares an open border to the south and west with the Republic of Ireland. At the 2021 census, its population was 1,903,175, making up around 3% of the UK's population and 27% of the population on the island of Ireland. The Northern Ireland Assembly, established by the Northern Ireland Act 1998, holds responsibility for a range of devolved policy matters, while other areas are reserved for the UK Government. The government of Northern Ireland cooperates with the government of Ireland in several areas under the terms of the Good Friday Agreement. The Republic of Ireland also has a consultative role on non-devolved governmental matters through the British–Irish Governmental Conference (BIIG).