perhaps UK will claim to not recognise EU as EU but only as individual countries to apply different rules?
They can't under Most Favoured nation clause. (MFN). If they drop all tariffs for Ireland to zero, the same will apply to the rest of the EU (they are one trading bloc, individual countries cannot sign trade deals) and all the rest of the countries under WTO. However it simply doesn't mean..if they are dropping their tariffs to zero for lamb (for example) for Ireland, they must offer the same to all countries, under they seek a special concession, which is rare.