The NI-Eire problem is more complicated than the way it has been represented here, there are 2 major problems, firstly, the Good Friday Agreement is actually not just an Agreement, it’s also an International Treaty and cannot be unilaterally changed by either side, one clause of the Treaty guarantees NI residents a right to redress from the ECHR, as the NI govt (well, the DUP) refuse to allow any proposals that treat NI residents differently it means as the Treaty stands the ECHR would have to remain for all UK citizens if a deal can’t be done.
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).