In case the United Kingdom (UK) leaves the European Union (EU) without any deal, World Trade Organisation (WTO) rules would apply for trade between the UK and any other country or trade bloc. Some British politicians have declared that under those circumstances, they would state that the UK will not perform checks at the Irish border. Under WTO rules, the UK must then also perform no checks on goods coming in from anywhere else. If the UK ignores those rules and imposes checks on other borders, they'll
probablycertainly get into trouble with WTO dispute settlement, which will take a while.
But more immediately, before the results of any dispute settlement, would under those circumstances third countries (non-UK, non-EU) be able to export to the UK by transiting goods through the Republic of Ireland and Northern Ireland — without importing those products to the EU?
The GATT, article V, provides for freedom of transit, but also states via the routes most convenient for international transit. If the UK were to check imports at some borders but not others (which appears to be what is being suggested), importers may wish to import via the Republic of Ireland (ROI)/NI border not because it is geographically most convenient, but because this border is not checked.
From the GATT:
Goods (including baggage), and also vessels and other means of transport, shall be deemed to be in transit across the territory of a contracting party when the passage across such territory, with or without trans-shipment, warehousing, breaking bulk, or change in the mode of transport, is only a portion of a complete journey beginning and terminating beyond the frontier of the contracting party across whose territory the traffic passes. Traffic of this nature is termed in this Article " traffic in transit ".
There shall be freedom of transit through the territory of each contracting party, via the routes most convenient for international transit, for traffic in transit to or from the territory of other contracting parties. No distinction shall be made which is based on the flag of vessels, the place of origin, departure, entry, exit or destination, or on any circumstances relating to the ownership of goods, of vessels or of other means of transport.
Any contracting party may require that traffic in transit through its territory be entered at the proper custom house, but, except in cases of failure to comply with applicable customs laws and regulations, such traffic coming from or going to the territory of other contracting parties shall not be subject to any unnecessary delays or restrictions and shall be exempt from customs duties and from all transit duties or other charges imposed in respect of transit, except charges for transportation or those commensurate with administrative expenses entailed by transit or with the cost of services rendered.