Republic of Ireland–United Kingdom border
Originally intended as an internal boundary within the United Kingdom of Great Britain and Ireland, the border was created in 1921 under the United Kingdom Parliament's Government of Ireland Act 1920. Prior to this, a separatist Irish parliament had been established in Dublin, which did not recognise the Government of Ireland Act, and was actively engaged in the Irish War of Independence. The Act was intended to deliver Home Rule in Ireland, with separate parliaments for Southern Ireland (which included three of the nine counties of Ulster) and Northern Ireland. Six of the thirty-two counties of Ireland were assigned to Northern Ireland, and the rest of Ireland comprising 26 counties to Southern Ireland.
Ireland–United Kingdom relations
Since at least the 1600s, all of these areas 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 1921 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, common language and close cultural and personal links mean political developments in both states often closely follow each other.