the EU only physically checks 3% of all non EU goods coming in
'The EU' doesn't check anything, the member states do.
so that is hardly going to be a problem on the Irish border
It's having a physical border, with checks, that's the problem, not the number of checks.
As for the idea up thread that somehow goods were going to be shipped to NI and then into ROI to get into the EU! How would that happen? Nothing comes into NI without coming through Britain, there are no direct routes to NI as far as I know.
Here's an example: if the UK drops its tariff on Widgets from the EU's 25% down to 0% then rather than import Widgets direct from the USA and pay 25% extra they could be imported into NI from the USA (and it doesn't matter whether they come direct from the US, through the UK or even through ROI) and then (in the absence of border controls) be shipped from NI to the ROI and then on to anywhere in the EU without paying that 25% import duty.
As for 'Nothing comes into NI without coming through Britain', you're talking rubbish.