Fair enough on your first point (it was a bit of a cheap shot from me!).You seem to be conflating goods and people. People are dealt with under the Common Travel Area, which predates the EU, the EC, the EEC and the ESCC.
I do take your point though that there could be some smuggling of lower standard goods in from the EU across the Irish border.
I think 'some' smuggling gives little indication of scale. We don't know yet but my guess is it will be considerable, with dire consequences for law and order and potentially for peace.