From the Cambridge dictionary:
"Already refers to things which have happened or which people think may have happened. Yet refers to things which have not happened or which people think may not have happened."
I think in this sentence, without context, either would be fine in English, it just depends on the expectations (and inflection) of the speaker.
(ie. the speaker might use "already" if they think it has been done, or if they are surprised to see it has been done already, and "yet" if they expect that it hasn't been done yet or are unsure. Wow, accidentally used each of the words in their corresponding examples, so I guess that goes to show how that works lol)
In terms of the translation, it would only be "yet" and not "already" because the translation from Irish is "yet". I haven't ever seen go fóill translated as "already", before, but I could be wrong.
Ar ullmhaigh tú an bia cheana? - "Did you prepare the food already?"
fós and go fóill don't mean "already".
It should also be pointed out that this distinction isn't new - either clairelanc3 was poorly taught, or didn't understand what was being taught. The distinction isn't between affirmative and negative sentences, it is between past and future, and events in the future have not happened (yet), and therefore "yet" statements use a negative term. When asking a question, you don't know the event has happened in the past (already) or will happen in the future (yet) so either can be used, but they are not interchangeable, as which word you use provides additional information about the speakers expectations.
Yeah I was pretty sure about fós and go fóill ≠ already, but I don't generally like to make definitive statements on Irish stuff here, because I've only been learning for like 3 months or something lol. Like, even if I know how a word is typically used, there are often plenty of other uses that I'm unaware of.
Thanks for the input!