The border between Ireland and Northern Ireland was established following an intense and complex struggle over the political status of Ireland in the early twentieth century in which Irish nationalists sought to achieve Irish Home Rule - the political independence of Ireland from British rule - and in which unionists, especially in the north-east of the island, sought to safeguard the continued political Union of Great Britain and Ireland. This political conflict was itself the product of a long and contentious history of British involvement in Ireland, in particular the establishment of a significant Protestant population in the north-east of the island with the Plantation of Ulster in the seventeenth century. It was shaped by the centrality of Protestantism to in the making of ideas of Britishness and loyalty to the British state and the strong associations between Catholicism and the form of Irish nationalism that became dominant in the late nineteenth century. By the early twentieth century, Ulster unionists were increasingly concerned about the close associations between Irish nationalism and Catholicism and sought to secure their place in the British state and the wider British empire by armed force if necessary.
The partition of Ireland into two separate political units as a comprise between the Irish nationalist demands for Irish Home Rule and unionist insistence on maintaining the Union was first considered in 1912 as Irish Home Rule was being debated in the British parliament in London and as unionists were increasingly mobilising to demonstrate support for the Union. The exclusion of some or all the counties of Ulster from Home Rule remained a possibility under the Third Home Rule Bill that was passed in 1914 but whose implementation was postponed until after the end of the First World War. However, the implementation of the Home Rule Bill was superceded by the republican Easter Rising of 1916, the growth of support for Irish political independence in Ireland rather than Home Rule, the Irish War of Independence from 1919 to 1921, and new legislation which sought to resolve the conflicting demands of Irish nationalism and unionism.
The Government of Ireland Act of 1920 which was passed during the Irish War of Independence legislated for the creation of Northern and Southern devolved governments within the United Kingdom and an inter-parliamentary Council of Ireland to work on limited areas of shared concern. The exclusion of three of the counties of Ulster – Donegal, Cavan and Monaghan - from the new Northern Ireland jurisdiction made up of six of the counties of Ulster – Londonderry, Fermanagh, Tyrone, Armagh, Antrim and Down - reflected unionist pressures to establish a region with a secure unionist majority. The new Northern Ireland Parliament was opened in June 1921. The Government of Ireland Act was passed before the end of the War of Independence and the rest of Ireland remained officially governed from London until the Anglo-Irish Treaty ended the conflict in December 1921. This Treaty upheld partition and granted dominion status to the South. Northern Ireland continued as a devolved political unit within the United Kingdom, and a Boundary Commission was proposed to consider amendments to the definition of the border between Northern Ireland and the new Irish Free State which was initially defined to correspond to county boundaries. The work of the Boundary Commission which began in November 1924 was the subject of much controversy, and mounting concerns over the destabilising effects of its relatively minor recommendations for change in the location of the border lead to the Triparite Boundary Agreement being signed in December 1925 – an agreement to suppress the report of the Boundary Commission, leave the border unchanged and discard the proposed all-island Council of Ireland. The creation of the new six county political unit created a large Catholic-nationalist minority in Northern Ireland and small but significant Protestant-unionist minorities in the Free State especially in the border counties of Cavan, Monaghan and Donegal.
While the location of the border has remained unchanged since it was established, the character and function of the border has shifted with changing economic and political developments over the course of the twentieth century.