The partition of Ireland (Irish: críochdheighilt na hÉireann) was the process by which the Government of the United Kingdom of Great Britain and Ireland 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 for both territories to remain within the United Kingdom and contained provisions for their eventual reunification. The smaller Northern Ireland was duly created with a devolved government (Home Rule) and remained part of the UK. The larger Southern Ireland was not recognised by most of its citizens, who instead recognised the self-declared 32 county Irish Republic. On 6 December 1922, a year after the signing of the Anglo-Irish Treaty, the territory of Southern Ireland left the UK and became the Irish Free State, now the Republic of Ireland.

The territory that became Northern Ireland, within the Irish province of Ulster, had a Protestant and Unionist majority who wanted to maintain ties to Britain. This was largely due to 17th-century British colonisation. However, it also had a significant minority of Catholics and Irish nationalists. The rest of Ireland had a Catholic, nationalist majority who wanted self-governance or independence. The Irish Home Rule movement compelled the British government to introduce bills that would give Ireland a devolved government within the UK (home rule). This led to the Home Rule Crisis (1912–14), when Ulster unionists/loyalists founded a paramilitary movement, the Ulster Volunteers, to prevent Ulster being ruled by an Irish government. The British government proposed to exclude all or part of Ulster, but the crisis was interrupted by the First World War (1914–18). Support for Irish independence grew during the war.

Irish republican party Sinn Féin won the vast majority of Irish seats in the 1918 election. They formed a separate Irish parliament and declared an independent Irish Republic covering the whole island. This led to the Irish War of Independence (1919–21), a guerrilla conflict between the Irish Republican Army (IRA) and British forces. In 1920 the British government introduced another bill to create two devolved governments: one for six northern counties (Northern Ireland) and one for the rest of the island (Southern Ireland). This was passed as the Government of Ireland Act, and came into force as a fait accompli on 3 May 1921. Following the 1921 elections, Ulster unionists formed a Northern Ireland government. A Southern government was not formed, as republicans recognised the Irish Republic instead. During 1920–22, in what became Northern Ireland, partition was accompanied by violence "in defence or opposition to the new settlement". The capital Belfast saw "savage and unprecedented" communal violence, mainly between Protestant and Catholic civilians. More than 500 were killed and more than 10,000 became refugees, most of them from the Catholic minority.The War of Independence resulted in a truce in July 1921 and led to the Anglo-Irish Treaty that December. Under the Treaty, the territory of Southern Ireland would leave the UK and become the Irish Free State. Northern Ireland's parliament could vote it in or out of the Free State, and a commission could then redraw or confirm the provisional border. In early 1922 the IRA launched a failed offensive into border areas of Northern Ireland. The Northern government chose to remain in the UK. The Boundary Commission proposed small changes to the border in 1925, but this was not implemented.

Since partition, Irish nationalists/republicans continue to seek a united independent Ireland, while Ulster unionists/loyalists want Northern Ireland to remain in the UK. The Unionist governments of Northern Ireland were accused of discrimination against the Irish nationalist and Catholic minority. A campaign to end discrimination was opposed by loyalists who said it was a republican front. This sparked the Troubles (c.1969–98), a thirty-year conflict in which more than 3,500 people were killed. Under the 1998 Good Friday Agreement, the Irish and British governments and the main parties agreed that the status of Northern Ireland will not change without the consent of a majority of its population.