Think of the border as a piece of rope strung over a canal that flows between NI and the Republic. At the moment, there's nothing to impede the free passage of boats. The level of the canal is the same on both sides of the rope.
A) Equalise the water
B) Build a lock
And anyone pretending there's a third is deluding themselves - and others.
(*indistinguishable from magic - nod to Arthur C Clarke)