Because ultimately it would be paid by Ireland citizens, not people who made a decision to slash Apple taxes. And this is not the right punishment.

Besides, most likely it didn't look like "Hey Apple, we will slash your taxes out of goodness of our heart". I bet Apple didn't protest when they slashed their taxes or even said something like "if you offer more attractive taxes, we will move some of our operations to your country instead of some other country" like all big companies in such cases. So they aren't innocent.

Only problem I see here is that most likely officials who allowed this won't be punished. In my opinion they should be (for "selling" to some company instead of serving citizens) together with a company (for accepting this, knowing that it's not exactly the right thing to do).