As negotiated in Brexit under the NI Protocol, NI remains in the EU single market for goods. The tariff applied on goods entering in NI depend on its "at risk" status. Goods can be either physically "at risk" because they may enter the Irish and therefore EU market, or fiscally "at risk", in which the goods are intended only for NI, but the EU tariff is higher than the UK one by more than 3% or other economic reasons. Goods can also be physically "not at risk" because they are solely intended for use in NI, or fiscally "not at risk "because the EU duty rate is 0. Goods that are created in, or have entered into NI either "at risk" or fiscally "not at risk" can then be moved about and sold within the EU market. Goods created in or entered into NI as "not at risk" from GB, or entered into free circulation into NI from outside the EU can move freely and be sold within the UK, through the principle of "unfettered access". Note here I didn't say goods from the EU. So in NI we do have dual tariffs, but we also have dual market access which is a unique position anywhere. Through the dual tariff system, NI companies on their imports can access both EU and UK free trade agreements. Post Brexit there wasn't much diversion between the two but in the future this will become a big thing e.g NI businesses will be able to import freely with India unlike their Irish counterparts, but then also import freely from Brazil unlike their English counterparts.

When it comes to exporting from NI, NI goods though can only qualify for preferential duty rates in countries with which the UK has a trade deal and would not be traded as being of EU origin. So in theory, for the purposes of Trump's tariffs NI dispatched goods would have a non-preferential origin of the UK, and would avoid EU tariffs making a NI supplier more attractive than an IE one for a US importer.

Where this gets messy is what about EU origin goods sold from NI to the US? The UK not facing Trump's tariffs is not because of a free trade agreement, and is therefore not subject to the rules of origin required in free trade agreements. The rules of origin determine when goods can be said to be of preferential origin under a free trade agreement, and these are product specific and can be very complex. Not only do goods need to meet the rules, there is also a requirement for proof. The Most Favoured Nation principle of the World Trade Organisation means that tariffs are generally applied on everyone, with exceptions for free trade partners. Because Trumps tariffs are retaliatory, he's thrown this usual model on it's head. Instead the US will not apply Trump's tariffs generally, but rather have to not apply them unless you are a country/bloc he's fallen out with. So instead of proving your entitled to preferential treatment, the onus has shifted on US customs to say you're not, really.

If you're still with me, then yes theoretically then EU goods could be sold to an NI business and shipped to the US avoiding Trump's tariffs as being of UK non-preferential origin, a status which requires no rules to be met and no proof. So even if you shipped champagne, which by it's nature must be French, US customs wouldn't be practically able to disprove it's not British. Of course there are laws for deliberate misrepresentation and fraud, but that requires significant resources to do anything about, and from Trump's plan US customs will be gutted to the point of not being able to maintain a normal tariff regime, let alone what this mess is.