- Between ROI and NI. Unacceptable and unenforceable since it would contravene the Good Friday Agreement and provide fresh air for the simmering sectarian conflict in NI
From a legal point of view, it is questionable whether it contravenes the GFA. The GFA is quite silent on the issue, and Customs matters are reserved to Westminister, so there's no problem with the NI Assembly.
The second point: absolutely. I'd fully expect the UK to return to the pre-1993 state of affairs, where only people doing legitimate business were bothering to report to the UK customs posts.
What would actually be smuggled now as opposed to pre-Brexit though?
So, let me answer this. Before 1993 (which was when the Single Market came into effect and customs posts were closed), the standard scam involved exporting goods on one side of the border and then not importing it officially on the other side. Let's take two scenarios:
- Lorry load of meat from the RoI. The meat is exported officially at the Irish border (in 1992, the customs post was in the middle of Dundalk), then the truck would simply cross the border at Killeen and vanish.
- Lorry load of televisions from the UK. The goods would be exported (and VAT returned) at the Newry customs post, which was on the NW side of Newry. The televisions would then be smuggled into the Republic, either by crossing the border at night when the customs posts were closed, or as was more common, they would simply be unloaded and reloaded within one of the many farms that biseect the border.
It wasn't such a big deal for the EU in those days, because the goods couldn't be exported from the UK/RoI without crossing a customs border. Now that there's the Single Market, a truck full of televisions can be smuggled anywhere in the EU because of the lack of controls between the RoI and the rest of the EU.
With VAT at 20% in the UK and 23% in the RoI, there is a huge incentive to smuggle in both directions. It gets even more serious when you think about excisable goods, such as fuel or cigarettes. Let's say that you export a tanker of fuel. You'll get quite a substantial refund on duties and VAT at the UK border, which can then be sold in various dodgy outlets in the RoI, and vice-versa.
Let's say that fuel is cheaper in NI. You buy a tanker of fuel at xx price, get roughly 60-70% back on export, then sell it for perhaps 20% off the pump price in the RoI. It's easy to see why the IRA were smuggling fuel with that kind of profit on offer.