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).

Northern Ireland was created in 1921, when Ireland was partitioned by the Government of Ireland Act 1920, creating a devolved government for the six northeastern counties. As was intended by unionists and their supporters in Westminster, Northern Ireland had a unionist majority, who wanted to remain in the United Kingdom; they were generally the Protestant descendants of colonists from Britain. Meanwhile, the majority in Southern Ireland (which became the Irish Free State in 1922), and a significant minority in Northern Ireland, were Irish nationalists (generally Catholics) who wanted a united independent Ireland. Today, the former generally see themselves as British and the latter generally see themselves as Irish, while a Northern Irish or Ulster identity is claimed by a significant minority from all backgrounds.

The creation of Northern Ireland was accompanied by violence both in defence of and against partition. During The Troubles in Ulster (1920–1922), the capital Belfast saw major communal violence, mainly between Protestant unionist and Catholic nationalist civilians. More than 500 were killed and more than 10,000 became refugees, mostly Catholics. For the next fifty years, Northern Ireland had an unbroken series of Unionist Party governments. There was informal mutual segregation by both communities, and the Unionist governments were accused of discrimination against the Irish nationalist and Catholic minority. In the late 1960s, a campaign to end discrimination against Catholics and nationalists was opposed by loyalists, who saw it as a republican front. This unrest sparked the Troubles, a thirty-year conflict involving republican and loyalist paramilitaries and state forces, which claimed over 3,500 lives and injured 50,000 others. The 1998 Good Friday Agreement was a major step in the peace process, including paramilitary disarmament and security normalisation, although sectarianism and segregation remain major social problems, and sporadic violence has continued.

The economy of Northern Ireland was the most industrialised in Ireland at the time of partition, but soon began to decline, exacerbated by the political and social turmoil of the Troubles. Its economy has grown significantly since the late 1990s. Unemployment in Northern Ireland peaked at 17.2% in 1986, but dropped back down to below 10% in the 2010s, similar to the rate of the rest of the UK. Cultural links between Northern Ireland, the rest of Ireland, and the rest of the UK are complex, with Northern Ireland sharing both the culture of Ireland and the culture of the United Kingdom. In many sports, there is an All-Ireland governing body or team for the whole island; the most notable exception is association football. Northern Ireland competes separately at the Commonwealth Games, and people from Northern Ireland may compete for either Great Britain or Ireland at the Olympic Games.

The region now known as Northern Ireland was historically inhabited by Irish-speaking, predominantly Catholic Gaels. It consisted of several Gaelic kingdoms within the province of Ulster. In 1169, Ireland was invaded by Anglo-Norman forces under the English crown, initiating centuries of foreign dominance. While English authority spread across much of Ireland, Ulster's major Gaelic kingdoms largely retained their autonomy, with English control limited to parts of the eastern coast.

During the Nine Years' War (1593–1603), an alliance of Gaelic chieftains led by Hugh Roe O'Donnell and Hugh O'Neill resisted English rule. Despite early successes and support from Spain, the alliance was ultimately defeated, culminating in the Flight of the Earls in 1607, when many Ulster nobles fled to mainland Europe. Their lands were confiscated and colonized by English-speaking Protestant settlers from Britain in the Plantation of Ulster, establishing a lasting Protestant community with ties to Britain.

The Irish Rebellion of 1641 began in Ulster, driven by demands to end anti-Catholic discrimination, achieve greater Irish self-governance, and reverse the plantations. The uprising escalated into an ethnic conflict between Irish Catholics and British Protestant settlers and became part of the wider Wars of the Three Kingdoms (1639–1653), concluding with the English Parliamentarian conquest

Subsequent Protestant victories in the Williamite-Jacobite War (1688–1691), including the Siege of Derry and the Battle of the Boyne, solidified Anglican Protestant rule in Ireland. These events are still commemorated by some Protestants in Northern Ireland. Many more Scots Protestants migrated to Ulster during the Scottish famine of the 1690s.