"They will eat fish and vegetables in the restaurant."
Translation:Íosfaidh siad iasc agus glasraí sa bhialann.
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”.)