While the UK could invoke article 21 of WTO (previously GATT) rules, claiming “vital national security” to allow goods from the Republic of Ireland tariff-free access into Northern Ireland, it is uncertain if this would ultimately be upheld by the WTO. There is, however, an alternative approach.
The UK could, simultaneously with the passing of the Brexit deal, also pass an Act committing it to granting Northern Ireland Freeport status at the end of the transition period if no free trade agreement with the EU had been signed. If a free trade agreement were signed, then no backstop would be needed because goods could flow freely across the border.
If no free trade deal were agreed, however, the granting of Freeport status to Northern Ireland would mean that goods from outside could enter tariff-free, unilaterally removing the requirement for a backstop. Northern Ireland would not need to be part of the Customs Union because the border between North and South would remain open to goods. Once it has left the EU, the UK will be free to set up Freeports wherever it wishes.
It would be entirely up to the Republic of Ireland and the rest of the EU, to decide how to respond to this UK initiative, but it would be hugely advantageous to Northern Ireland and would boost both cross border traffic and inward investment. The DUP would almost certainly endorse it enthusiastically, given the shot in the arm it would impart to the Northern Irish economy.
There is quite a good possibility that such a commitment would bring the DUP behind the Brexit deal, and allay many of the misgivings that some MPs have concerning the backstop situation, and allow them to support the deal. There is an even stronger possibility that such a commitment would encourage the EU to agree a free trade deal that would make it unnecessary before the end of the transition period.