And why not a united Ireland?

The Border in Ireland has never made democratic sense. It was drawn to maximise the "little Orange empire" for the Protestant-Unionists of the north east in the 1921 partition of Ireland. It has been a running sore for almost a century. To this day, along almost all its length, the majority of the population on the Northern side of the Border is "Catholic", Irish-Irish rather than British-Irish in its identity. The Border makes no political or human sense now. For a long time there have been no border checks.