They can't.
The people living there VOTED to stay part of the UK.
Which is why the communist terror group IRA continues to attack them. After all a community freely choosing is anathema to them.
The irish and northern irish cannot have their cake and eat it too, i would imagine. At some point they will need to determine if an open border is more important than staying one one side or another of of the eu fence, or vice versa.
As for who enforces a border, perhaps ot will be the uk since otherwise they will have an undicumented immigrant aka foreign terrorist issue from ireland sooner or later with an open border. Either that or the brexit pain for the gain in sovereignty and security ultimately will be in vain.
Ok what am i missing...