Ireland–United Kingdom relations

Since at least 1542, England and later Great Britain and Ireland have been connected politically, reaching a height in 1801 with the creation of the United Kingdom of Great Britain and Ireland. About five-sixths of the island of Ireland seceded from the United Kingdom in 1922 as the Irish Free State. Historically, relations between the two states have been influenced heavily by issues arising from their shared (and frequently troubled) history, the independence of the Irish Free State and the governance of Northern Ireland. These include the partition of Ireland and the terms of Ireland's secession, its constitutional relationship with and obligations to the UK after independence, and the outbreak of political violence in Northern Ireland. Additionally, the high level of trade between the two states, their proximate geographic location, their common status as islands in the European Union until Britain's departure, common language and close cultural and personal links mean political developments in both states often closely follow each other.

Irish and British citizens are accorded equivalent reciprocal rights and entitlements (with a small number of minor exceptions) and a Common Travel Area exists between Ireland, United Kingdom, and the Crown Dependencies. The British–Irish Intergovernmental Conference acts as an official forum for co-operation between the Government of Ireland and the Government of the United Kingdom on matters of mutual interest generally, and with respect to Northern Ireland in particular. Two other bodies, the British–Irish Council and the British–Irish Parliamentary Assembly act as a forum for discussion between the executives and assemblies, respectively, of the region, including the devolved regions in the UK and the three Crown dependencies. Co-operation between Northern Ireland and Ireland, including the execution of common policies in certain areas, occurs through the North/South Ministerial Council. In 2014, the UK Prime Minister David Cameron, and the Irish Taoiseach Enda Kenny described the relationship between the two countries as being at 'an all time high'.

Both Ireland and the United Kingdom joined the European Union (then the European Communities) in 1973. However, the three Crown dependencies remained outside of the EU. In June 2016 the United Kingdom held a referendum in which the majority of the voters voting in the referendum voted to leave the European Union. Brexit became effective on 31 January 2020, with a deal being reached on 24 December 2020, keeping Northern Ireland in the European Union Single Market for goods and keeping a free border between the Republic of Ireland and Northern Ireland. Relations between both sides have been poor since the requested implementation of the Northern Ireland Protocol on 1 January 2021, which is strongly opposed by British citizens of that region, who view it as a "political assault" on their democracy, by the EU and Irish government. As a consequence of the protocol, Northern Ireland has been without a fully-functioning executive government since 3 February 2022, after the DUP dissolved Stormont in protest of the protocol.

The three devolved administrations of the United Kingdom, in Scotland, Wales and Northern Ireland, and the three dependencies of the British Crown, the Isle of Man, Jersey and Guernsey, also participate in multilateral bodies created between the two states.