Any understanding of Brexit's impact on Northern Ireland necessarily needs to begin with an understanding of the historic relationship between Britain and Ireland. This relationship is grounded in protracted conflict, including during the early twentieth century, when a period of anti-colonial agitation by Irish nationalists and counter-mobilization by unionists in Ulster ended with partition, and the creation and consolidation of two new states on the island of Ireland: the independent Irish Free State and Northern Ireland, which remained a part of the Union. Partition was intended as a solution to the “Irish question” which had been an obdurate feature of British politics since the late nineteenth century. The creation of the Irish Free State (later the Republic of Ireland) precipitated a marked shift in the character of the British–Irish relationship. What had previously been a colonial relationship between Ireland and Britain changed to become (in theory at least) a relationship of sovereign equals. However, the birth of an independent Irish state in the midst of violent conflict would leave an enduring mark. In particular, the legitimacy of the border separating the Free State from the North was a key issue at stake in the Irish Civil War (1922–23), and it would remain essentially contested.

Shortly after the foundation of the Irish Free State, the Irish party system crystallized into an unusual form where political parties were based primarily on a constitutional rather than a socio-economic or class cleavage. As Ó Beacháin (2018: 5) notes, even to this day: “Ireland's party system remains a product of the civil war of 1922–1923”. Right from the state's foundation therefore, the constitutional question was embedded in the Irish political system, shaping party competition and framing state structures and institutions.

Thus, Irish politics has been implicitly framed by consideration of Northern Ireland. This was evident in the early years after the foundation of the Irish Free State, following the signing of the Anglo-Irish Treaty in 1921, when politics, communities and even families were torn apart by the pro- versus anti-Treaty dichotomy.