Disclaimer: I’m neither a lawyer nor involved in international trade, this is based on what I assume to be a massively over-simplified understanding of world trade rules.
The UK wants to leave the EU customs union. They are allowed to do this.
Leaving the EU customs union necessarily means that any goods crossing from the UK into the EU customs union will need to pass through customs checks. To stop this, the UK and the EU would have to agree (some sort of) trade deal — the UK does not have the power or the right to prevent this alone.
The UK has an agreement with the Republic of Ireland, the Good Friday Agreement, which reportedly means neither party is allowed to install customs checks on the border (does it? I’ve seen people claim that but I’ve not seen details). The terms of the Good Friday Agreement do not make it illegal for the UK to leave the EU customs union regardless, as there are ways to meet both requirements:
- The UK could blindly accept all goods from the EU.
- The border between the EU customs union can be somewhere other than the Northern Ireland/Republic of Ireland border:
- The Republic of Ireland could leave the EU.
- Northern Ireland can become independent from the UK.
- Northern Ireland can remain within the EU customs union even after the rest of the UK leaves.
- There could be a new internal border within Northern Ireland.
All of these have problems, the question is which problems you’re willing to deal with.
The UK could blindly accept all goods from the EU
World Trade Organisation rules say that (outside of free trade agreements), all trade must be on the same terms as the most favoured nation: if you drop tariffs for one nation, you must drop them for all; if you accept goods from one nation without checking them, you must do so for all; etc.
If the UK accepts all goods from the EU without checking them, then the UK is obliged to also accept all goods from everyone else, also without checking them.
This does not oblige the EU to accept goods coming from the UK, but would allow the UK to be following all the rules.
The Republic of Ireland could leave the EU
The Republic of Ireland has no desire to do this. (It might happen).
Northern Ireland can become independent from the UK / Northern Ireland can remain within the EU customs union even after the rest of the UK leaves
The UK government will probably fall if this happens. (It might happen).
There could be a new internal border within Northern Ireland
I don’t know enough about the politics of Northern Ireland to be sure, but isn’t this sort of thinking exactly what caused all the fireworks between India and Pakistan, between and inside seemingly half of the nations in Africa, the reason the Berlin Wall was a symbol of the Cold War, and one of the main reasons Northern Ireland didn’t immediately become peaceful the moment the Republic of Ireland became independent from the UK?
Obviously, the only acceptable course from the perspective of the UK government is to try to force the Republic of Ireland to leave the EU. Fortunately for the Republic of Ireland, the UK government is wildly incompetent.
Oh, one more problem:
The Good Friday Agreement wasn’t just with the Republic of Ireland, it also involved the total disarming of all paramilitary groups. The paramilitary groups reportedly dragged their feet with that disarming, so even if the governments agree, there may be an undesirable fan-feces interaction.
Advice? I suppose you could invest in armoured reinforcement manufacturers, but this isn’t going to be fun for anyone.