Republic of Ireland–United Kingdom border in the context of "Northern Ireland"

The impact of Brexit on the Irish border and its adjacent polities involves changes in trade, customs, immigration checks, local economies, services, recognition of qualifications, medical cooperation, and other matters, as it is the only land border between the United Kingdom and the European Union.

After the UK Parliament voted to leave the European Union, all parties said that they want to avoid a hard border in Ireland, due particularly to the border's historically sensitive nature. Border issues were one of three areas of focused negotiation in the Withdrawal Agreement. Following the United Kingdom's exit from the European Union on 31 January 2020, this border is also the frontier between the EU and an external country.

The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of the continental mainland. It comprises England, Scotland, Wales and Northern Ireland, with a population of over 69 million in 2024. The UK includes the island of Great Britain, the north-eastern part of the island of Ireland, and most of the smaller islands within the British Isles, covering 94,354 square miles (244,376 km). It shares a land border with the Republic of Ireland and is otherwise surrounded by the Atlantic Ocean, the North Sea, the English Channel, the Celtic Sea and the Irish Sea, while maintaining sovereignty over the British Overseas Territories. The capital and largest city of both England and the UK is London; Edinburgh, Cardiff and Belfast are the national capitals of Scotland, Wales and Northern Ireland.

Britain has been inhabited since the Neolithic. In AD 43 the Roman conquest of Britain began. The Roman departure between 383 and 410 was followed by Anglo-Saxon settlement beginning around 450. In 1066 the Normans conquered England. After the Wars of the Roses, the Kingdom of England began to flourish, resulting in the 16th-century annexation of Wales and the establishment of the British Empire. Over the 17th century the role of the British monarchy was reduced, particularly as a result of the English Civil War. In 1707 the Kingdom of England and the Kingdom of Scotland united under the Treaty of Union to create the Kingdom of Great Britain. In the Georgian era the office of prime minister became established. The Acts of Union 1800 incorporated the Kingdom of Ireland to create the United Kingdom of Great Britain and Ireland in 1801. Most of Ireland seceded from the UK in 1922 as the Irish Free State, and the Royal and Parliamentary Titles Act 1927 created the present United Kingdom.

Newry (/ˈnjʊəri/; from Irish An Iúraigh) is a city in Northern Ireland, standing on the Clanrye river in counties Down and Armagh. It is near the border with the Republic of Ireland, on the main route between Belfast (34 miles/55 km away) and Dublin (67 miles/108 km away). The population was 27,913 in 2021.

Newry was founded in 1144 as a settlement around a Cistercian abbey. In the 16th century the English dissolved the abbey and built Bagenal's Castle on the site. Newry grew as a market town and a garrison, and became a port in 1742 when the Newry Canal was opened, the first summit-level canal in Ireland. A cathedral city, it is the episcopal seat of the Roman Catholic Diocese of Dromore. In 2002, as part of the Golden Jubilee of Elizabeth II, Newry was granted city status along with Lisburn.

The Irish Sea border is an informal term for the trade border between Northern Ireland and Great Britain. It was specified by the Ireland/Northern Ireland Protocol of the Brexit withdrawal agreement (February 2020), was refined by the Joint Committee in December 2020, and came into effect on 1 January 2021 following the end of the Brexit transition period. As a result of the Agreement, Northern Ireland remains aligned to the European Single Market in a limited way for goods, whilst remaining part of the United Kingdom customs territory and the UK internal market. Its effect is that the need for customs checks on the Irish border has been avoided, and a hard border has not been re-established.

This Irish Sea border was the option taken by Prime Minister Johnson in October 2019 to break the impasse of the "Brexit Trilemma" (of three competing objectives: no hard border on the island; no Irish Sea border; and no British participation in the European Single Market and the European Union Customs Union: it is not possible to have all three.)

The Ulster-Scots Agency (Ulster-Scots: Tha Boord o Ulstèr-Scotch) is a cross-border body for Ireland which seeks to "promote the study, conservation and development of Ulster-Scots as a living language, to encourage and develop the full range of its attendant culture, and to promote an understanding of the history of the Ulster-Scots [people]."