Only if the joined countries don't implement some kind of differentiated tax on products and services commiting from outside the list.

No, it just matters if countries are willing to implement consequences for non-membership. And that's frankly a pretty trivial thing from a political and technical perspective, so I'm assuming it'll come. Various countries already have lists, e.g. the EU has lists for 'non-cooperative third-countries', which can be used by member states to add witholding taxes for transactions with these countries in-particular as a preemtive measure to reduce aggressive tax avoidance.

Ireland is not joining this agreement for no reason, they're doing it because non-membership will hurt them more in the end.

That's easy to fix, at least technically: punish financially all the countries who don't sign this. Just requires willingness to do this.