"They will eat fish and vegetables in the restaurant."
Translation:Íosfaidh siad iasc agus glasraí sa bhialann.
I think that iasc is used here as a mass plural (“fish” as in “fish flesh”) rather than as an individual fish.
Yes that's what I thought but I felt that that was a hangover from English because we don't use a plural of fish in English. As I am not a native Irish speaker I didn't know what is more natural to say 'as Gaeilge' in the above context - iasc or éisc.
If it’s a hangover from English, it’s been in Irish since at least Dinneen’s time. Feoil is also treated as a mass plural, e.g. An itheann tú feoil Dé hAoine?, despite the existence of a countable plural feolta. (Of course, English has a countable plural “fishes” as well a mass plural “fish”.)