Remember:
1. There are no goods borders between Ireland and the rest of the EU; and
2. There are no goods borders between Northern Ireland and the rest of the UK.
That's where it's by far the most efficient and effective to control what enters a country.
Behind the border checks and audits are less effective and spread the bureaucratic burden.
Because previously, the place where RoI / NI meet didn't mark the outer edge of two different trade areas. By the time anything got there, it had already passed through screening at the EU SM/CU outer edge.
Cool.
Except the EU defends the integrity of the SM/CU's borders and will demand the RoI does too. Remember: borders have two sides.