The present border in Ireland is geographically arbitrary but not accidental. Historically, the province of Ulster comprised nine counties, but in the early 20th century, as clamour for Irish independence grew, it had too many Catholics in it for unionist comfort. In 1920, when Parliament partitioned Ireland, six counties were chosen to comprise the new self-governing Northern Ireland. Even at the time of partition, about one third of Northern Ireland’s population was Catholic. It is approaching half now.
In many respects partition made sense, but it can hardly be seen as an unqualified success. It is notable that the Good Friday Agreement of 1998, which formally superseded the 1920 Government of Ireland Act, contains no provision for any future repartitioning of the island. So long as there is a unionist majority, Northern Ireland remains in the United Kingdom. If that ever changes, it passes in its entirety to the Republic of Ireland. Ireland is supposed to be done with partition.