From Gramadach na Gaeilge:
If a verb should be incorporated (e.g. "who says that?"), there is still the need of a real subject, which is then replaced by a relative clause with the verb: e.g. cé a deir sin? = who says that?, lit.: "who is(it), that says that?". cé a rinne sin? = who did that?
It's under the "Other Words" section.