If the UK as a whole remains in the Customs Union,there is no problem and no need for customs checks between any member states and the UK. EU inspectors and UK inspector at either factories, and the main ports can do the business.
If the NI backstop functions only, the customs checks are at Holyhead, Pembroke,Liverpool and Clanryane.
Spot checks along the M1 near Carrickaron,would be ok. There are Sidings in place. But extensive 100% checking would grind traffic to a halt ,and would put liquid milk delivery , sandwiches mushrooms and duck to Northern England in jeopardy.
If the NI backstop dissapears ,and it could in the event of a no deal,along with the British cabinet, .then all bets are off. The EU are not bluffing on this matter. It is not as I have seen " a straw man argument. "
It might be instructive for you Britons to open up the Google map of the border area. And note the interlinking of roads. It is usual when borders are drawn to have natural features eg. Mountains, rivers ,fjords as the boundaries. Except for the 10 miles from Carlingford Lough towards Newry, that is not the case with NI. The roads go between NI and RoI,with only the wider dotted white line on the NI roads to distinguish the sections. The border goes between rooms in houses, between sheds and outhouses, etc. And when you look at the Google Map, remember it does not show boreens.. narrow lanes for cattle movement.