Some commentators on Brexit this week seem to have forgotten paragraph 46 of the December agreement, to which the UK as well as the EU signed up. It states:
The commitments and principles outlined in this joint report will not pre-determine the outcome of wider discussions on the future relationship between the European Union and the United Kingdom and are, as necessary, specific to the unique circumstances on the island of Ireland.
In other words, HMG can't use the Irish border issue as leverage to get an unusually favourable deal with the EU for the UK as a whole.
That needed to be spelled out, since some in HMG regarded the border as the key that would allow the UK to have its cake and eat it..
That would allow it to do trade deals with Trump, and diverge in terms of regulations, and still mysteriously be able to benefit from frictionless trade with the EU, thus preserving its supply-chain based industries.
Some in HMG were even incautious enough to say as much publicly, for example Johnson when in Dublin.
It was cynical, but also completely transparent.
Hence paragraph 46.