A number of EU rules need to be agreed to unanimously: new members, treaties, taxation, IIRC.
Ireland (the Republic, not NI) has repeatedly stated its concerns with the re-introduction of a hard border, a concern the backstop was designed to avoid.
The new UK Prime Minister Boris Johnson talks a fair bit about engaging with the EU to remove that backstop and talking to Merkel and Macron. But ultimately, is Ireland fully capable of blocking a Brexit deal that it considers unfavorable, just by exercising a veto? In which case, why the emphasis on discussing the backstop with the EU as a whole or Germany and France, if Ireland can pull the plug at any time?
Does Ireland have an explicit and formal veto power on amending the backstop agreement if it feels it goes against its interests? If so, seems to me as if Boris Johnson needs to sell whatever he's selling to them first and they've been unambiguously critical of adjusting the backstop.
Or would such arrangement fall under EU majority rules, rather than needing unanimous approval?
As I recall, the EU-Canada trade deal came very close to being derailed due Wallon (Belgium) objections concerning either cattle or dairy. This wasn't resolved until Belgium got some concessions and everyone more or less acknowledged Belgium's right to veto. I can't see anyone forcing Ireland to swallow concessions about something that's as important to them.
Note that I am not talking about future treaties the EU and the UK might enter into past Brexit. That's an unanimity-required domain.