Who will implement it?
The RoI will go back to what they did before 1993. That's not the problem - the problem is that any return of controls on the Northern Irish side could be a threat to the peace process. People categorically don't want controls to return there, and I can't imagine HMRC will have many willing volunteers to go and man a customs post in South Armagh.
Ireland has nothing to worry about, because their customs controls were never the target of attack between the foundation of the Free State and 1993. It was the UK customs stations that came under repeated and sustained attack throughout their existence.
The question is, how will the U.K. protect it's border with the E.U.?
They won't. They've already announced no controls at the major ports because they simply don't have the capability of carrying them out.
Ireland is preparing for no-deal and the return of customs controls. The UK hasn't announced anything in respects to the Irish border, because they simply have no plan. There's a high chance that "taking back control" will actually mean "a free for all on exports from the EU to the UK".