Hard border is an EU tactic to stop Brexit.
It is an impossibility anyway. There are about 200 crossing points.

If the EU thinks it needs one it will have to make Eire provide it.
Good luck to them with that.
They will also have to provide security. Whatever they pay them will not be enough.

If no hard border is created, the worse that could happen would be a bit of smuggling. Just invest the money in extra policing instead of building 200 Alamos.