"Cé a itheann sicín?"
Translation:Who eats chicken?
It's because a relative clause must be used to introduce the verb.
From Gramadach na Gaeilge:
Syntax of interrogatives:
Interrogatives contain for the most part some (invisible) form of the copula or, better said, the copula is included in the formation of the interrogative. Interrogative particle and intentional copula comprise the copular clause.
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?