Plantinga says that in order to say that theism is irrational, one must first show that theism is false or that Plantinga’s own model of warranted religious belief is false. Hence, Plantinga is saying that the question of rationality is not independent of the question of truth, according to his model.
One problem I have is that I’m not sure I would just plainly say “theism is irrational”. I would say theists themselves can be irrational or rational, which has more to do with persons. For instance, I would not say that Aquinas was irrational in his belief in God. On the other hand, I would say that someone (some theist) is being irrational if they say they believe in God only because it feels good or because they flipped a coin, and they are still being irrational even if God does exist (or doesn’t exist). And, some theists could presumably be (and are) irrational in their belief in God, even if Plantinga’s model is true. For instance, some theists’ cognitive faculties, as Plantinga words it, could be broken and that’s why they have come to believe that God exists based on dubious reasons.
So, I’m not sure one would have to show that Plantinga’s model is false (or that theism is false) in order to say theism is irrational (whatever that means). If we have no good reason to accept Plantinga’s model as being true, then we should not believe that it is true (which is not the same thing as saying it is false). And once again, I think there is a distinction between “theism is irrational” and “some theists are irrational”. I don’t see the latter as being necessarily tied to theism being true or Plantinga’s model being true.
Maybe Plantinga’s response to all of this is to grant that a theist (or theism) can be rational if God exists, not necessarily that all theists are rational.