I don't know what mode he is operating in. Kantians and their looser relatives (Rawls, presuppositionalist apologists), will usually at least admit that rational has to be adjudicated by some principal of consistency. In Kant, for instance, practical reason requires obedience to the categorical imperative, which looks very much something like the principle of non-contradiction for ethics. Given his approach, Plantinga probably means that atheists are irrational in the sense that they end up being internally inconsistent in a number of ways.

I'm fine as far as this goes, I suppose, but much like Hegel I want to actually go ahead and start swimming rather than standing by the pool and discoursing about what does and does not count as swimming. Knowing has a definite metaphysical character, as does the world, and it is not the mere consistent adjudication of an infinite number of givens. That approach to this problem ends up merely being a via negativa: what you wanted to say is ultimately what reason is and what the world is in their most general character, but that would be metaphysics, which he has soundly rejected, either explicitly or tacitly.