What do you mean? Whose irrationality?

It varies, and I might not even know. For example, if the arrangement of signs on a particular street intersection causes unnecessary traffic congestion, I might call it an irrational arrangement. In doing so I'd be presuming that whoever chose that arrangement intended to minimize traffic congestion, or at least asserting that they ought to have intended that. But I might have no idea who chose the arrangement. (I might also be wrong, but that's beside the point.)

But that said, and speaking very roughly: irrationality on the part of the most proximal agent(s) who was (were) capable of making a different choice.

Isn't it more straightforward (it's there among the 'virtues of rationality' no?) to just not call things 'rational' if they do not involve thinking?

Yes, it is.

For example, what I just described above is a form of metonymy... describing the streetsign arrangement as irrational, when what I really mean is that some unspecified agent somewhere in the causal history of the streetsign was irrational. Metonymy is a common one among humans, and I find it entertaining, and in many cases efficient, and those are also virtues I endorse. But it isn't a straightforward form of communication, you're right.

Incidentally, I suspect that most uses of 'rationality' on this site (as well as 'intelligence') could be replaced by 'optimization' without losing much content. Feel free to use the terms that best achieve your goals.