Unlike the Free State after 1921, Northern Ireland did not experience a civil war. The 1920 Government of Ireland Act had partitioned Ireland and established parliaments for both Southern and Northern Ireland (the six Ulster counties of Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone. The other three Ulster counties – Donegal, Cavan and Monaghan – were placed in Southern Ireland). Northern Ireland was a new and unique part of the United Kingdom, in area 5,452 square miles (slightly larger than Yorkshire and less than one-fifth of the area of Ireland) with a population of 1.256 million (in 1926) composed of a 2:1 Protestant majority over Catholics which was translated politically into a permanent Unionist supremacy. Between 1929 and 1968 in the Parliament of Northern Ireland, Unionists held never fewer than thirty-four of the fifty-two seats, and never less than two-thirds of the Northern Ireland seats at Westminster.

The first general election for the fifty-two-member Parliament held in May 1921 returned forty Ulster Unionist Party MPs. In fact the Ulster Unionist Party won every general election and formed every government until the Parliament was suspended in 1972. Its connections with the Orange Order were always close: every single Northern Ireland prime minister was a member of the Order. Unionists had consistently argued for the continuation of the union between Ireland and Britain, and had not sought the creation of Northern Ireland but had accepted it, in the words of Sir James Craig, its first prime minister, as ‘a supreme sacrifice in the interests of peace’.