Thing is, short of some serious EU money, nobody will ever pay for it. In the worst days of the Troubles the border was as made up as any line drawn on a map. There were next to no dedicated patrols by either side and very little in the way of checkpoints or fortification. In fact you were much more likely to get stopped by a Provo for PR purposes.
The three interested parties (UK, RoI and EU) are simply using the Good Friday Agreement as cover for something they never intended to do anyway, and in fact never really did do. It would be a total farce if they did. And possibly a lethal one once the dissidents had something to play off.