Putin needs to at least be able to use the pretext of an attack being reasonable for defensive reasons. He can paint Ukraine as being defensive (NATO/EU are dangerous to Russia, Neo Nazis are terrorising Russians in Ukraine, Ukraine are killing people in the Donbass, etc, etc) he can't with Ireland. He needs to choose somewhere which he can paint as a threat but which also isn't going to cause a massive retaliatory attack.
Sweden/Finland joining NATO, Georgia wanting EU membership are probably the two he'd point to at the moment. Even then it would need to be at a stage in the war where it seems unwinnable as he would only do it in the hope that he would scare the west into backing down.