The other problem is if the UK was to go no deal and move to WTO trading, WTO rules do not allow discrimination in trading, if the UK allows border free access without restriction to goods from Eire/the EU, it must also give free (zero tariff) access to goods from every other country in the world who it trades with (its known as the “most favoured nation” rule), which isn’t something the UK wants (or will allow).
They can however use standards as a trade barrier - allowing EU goods in whilst keeping goods from other countries out if it does not comply with the standard set. (eg US beef can be banned because of the use of antibiotics)
