Translation:Who are they?
why is there an h in iad? I got that we use iad with Is, but siad with the rest... but hiad? Why?
Because cé ends with a vowel sound. One thing to notice is that Irish usually doesn't like vowel sounds coming together (Exception includes the masculine possessive pronoun).
so there's no copula in this sentence? If I wanted to say "who is she", do I say "Ce i"??
Cé hí /cérb í, but yes. The copula is invisible here.
From Gramadach na Gaeilge:
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.