That's not an EU decision as you're suggesting there.

Ireland isn't some vassal state. It's an equal partner.

Ireland itself doesn't want to cut Ireland off from the EU.

It's not for the EU to decide something like that. It's an Ireland decision, and in fact the power to do so resides with Ireland.

If Ireland had wanted to align with the UK and separate from the EU, they could have, and they still have the power to do so. Of course they don't want to, why would they. They've benefited enormously from EU membership, and one of those benefits now is that the EU often protects the interests of Ireland - as determined by Ireland - in negotiations with the UK.

It is important to remember that the EU is not actually a top down system, even though it can seem that way. Any member country is free to leave simply by giving notice, and cease to partake of the benefits of membership. The EU's laws are clear on this.

Eh? No, it couldn’t; Ireland would obviously veto that. Outside of the fevered imagination of Boris Johnson, that was never an actual option.

I think there was a Brexiter theory that Ireland would do this voluntarily, and become a sort of UK dependency, but this was never at all realistic.

I wouldn’t discount the possibility that some major brexit figures actually believed this would happen and were surprised that it didn’t, because they’re, well, not winning any prizes for competence (you may remember David Davis believing that the UK could make a trade deal with Germany, say…) But no-one serious ever bought this concept.