Hard Brexit with a Belfast-sized hole in the EU customs. IE, goods may flow freely between NI and Ireland as if NI were still in the EU, and may flow freely between NI and the rest of the UK because they're the same country.
That's pretty much the plan. UK and ROI have travel and trade agreements that pre-date the EU. The EU has Frontex based in a Warsaw Tower of Power and a budget north of 300 million. And possibly some confusion about exactly what their roles and responsibilities are other than 'assisting' EU members with enforcing the EU's border policy.
And they have elite 'Rabid Border Intervention Teams', which means Irish paramilitaries on both sides of the border might end up hunting RABITs who're chasing smugglers. From the UK pov though, smuggling has long been a money spinner for those paramilitary groups, so there's still plenty of border surveillance and intelligence.
Biting the hand that feeds IT
