There was some discussion of this on TV a few weeks ago - I think it was Paxman quizzing Theresa Villiers. He was saying that it was infeasible that the UK would have an open border/non-border with the Republic of Ireland, and yet any EU citizen/resident could come to Ireland and just walk across the border into the UK (and then on to the mainland by other means).
Villiers was adamant that there would be no border control with the Republic of Ireland.
I don't know what the "correct" answer is, but there clearly is a problem here. Certainly I believe that the "regionalism" promoted by the EU is a reason there is less friction in Northern Ireland, or at least the reason it comes to the surface less often. Re-imposing border controls seems likely to inflame the situation rather than to support the current peace accord.