In Irish, the copula is implied with question words. Because it is still a copular structure, however, you need to make it a relative clause if you're using another verb, thus the relative particle a is needed.
Compare with Cé hé (who is he?) which doesn't have the second verb, and thus doesn't require a relative clause.
While the Ulster pronunciation of "cá" might be rendered "kah", it's not a very accurate rendition, and doesn't capture the "length" of the "a" sound accurately. "kah" is completely wrong for non-Ulster pronunciations of "cá" - http://www.teanglann.ie/en/fuaim/c%c3%a1
Even in Ulster Irish, "cé" not pronounced with a noticeably "slender c" - "kay" is a better rendition than "kyay". (It is technically a slender "c", but "ky" does not approximate that sound very well).
Like most people, I don't know the ins and outs of IPA, which I presume is the reason that you wrote "kah" and "kyay" rather than IPA representations. Most people pronounce "kyay" in a way that is nothing like "cé" - "kay" is a closer representation of the sound of "cé", even though it is not strictly accurate either, but the "slenderness" of the "c" in "cé" is barely noticeable in ordinary speech, and someone who has to ask for pronunciation advice would not know that, and would be mislead by "kyay".
Similarly, "kah" might look like a reasonable representation of the Ulster pronunciation of "cá", but what you are actually hearing is an Ulster pronunciation of "kaw". The IPA engine at abair.ie renders them all as /kaː/ even though the actual delivery of that sound by someone with an Ulster accent will be noticeably different (How does that happen with the IPA? Beats me!).
That is indeed odd. To my ears, the Ulster 'cá' is /kaː/ or /kæː/ while the Munster one is /kɑː/, and the Connacht is closer to /kɑ͡ʌ/.
Being someone who knows the IPA quite well, I learned to pronounce Irish with this: https://en.wikipedia.org/wiki/Help:IPA_for_Irish Now that I know the dialects can pronounce things quite differently, I will look elsewhere for accurate guide.
Thanks for your help!
The "a" in "cé a itheann iasc?" is more like "that" - "who is it that eats fish?"
While the negative form of "who eats fish?" is "who doesn't eat fish?", using "does" in "who does eat fish?" is not necessary, and it's presence makes the question very emphatic. Depending on the context, you might translate that emphasis with "cé hiad a itheann iasc?" or other contrasting forms.
One of the other items in this lesson asks for a translation of Céard a itheann madraí? and expects as an answer "What do dogs eat?". But if one were to parse the current sentence in parallel with this example one would get "Who do fish eat?". I tried this, and of course got the raspberry. But I'm still scratching my head over the underlying grammatical structure here.