While the UK could make any decision on goods travelling from Ireland into Northern Ireland – and therefore the UK – Ireland would need to set up a border control for goods moving in the opposite direction, because any goods moving from the UK into Ireland would be subject to the European customs code, said EU customs and international trade lawyer Eric Pickett.
Goods travelling from the Republic of Ireland into Northern Ireland would need an export declaration which would be checked at the border.
The UK could decide not to have a border control in the other direction, said Michael Lux, also an EU customs and international trade lawyer.
"If you don't feel you want to recover VAT and customs and excise duties, you are of course free not to do that, but Ireland is obliged to," he said.
However, evasion of taxes on goods coming into the UK is likely to become a serious issue if no controls are in place, he said.
It may be possible to put an agreement in place so that Ireland informs the UK of all exports, to allow it to recover VAT and excise duties, he said.
"We have had this request from Russia, as there is a lot of fraud there. However we have always refused this due to the [trade] secrets that we felt should not be shared outside the EU. So it's an interesting question, whether this information can be shared with the UK, which is not … so outside the EU," he said. "I do not dare to answer this but it would be problematic."
Guy Lougher of Pinsent Masons, the law firm behind Out-Law.com said: "The exchanges at this select committee on Northern Ireland affairs further illustrate the potential practical implications of Brexit for businesses operating north and south of the border. And what is very clear is that although there is a clear will to find a solution amongst the UK and Irish governments, the issues will need resolving at the EU / UK level. But even with the requisite political will on both the EU and UK sides it will be problematic to avoid customs formalities."