I have submitted at least two reports for this question, but they haven't addressed it yet...
Iasc na bialainne and Iasc na mbialann should both be accepted for Iasc na __.
This is a Duolingo problem, not an Irish course problem. The “select the missing word” type of exercise is just a presentation variant of the usual “translate this text” exercise, and thus there is only one set of correct answers based upon the correct translation solution, even though the missing word variant can offer more than one grammatically correct answer. The only realistic workaround would be to eliminate the simpler exercises, to ensure that only one grammatically correct answer could be presented in the missing word variant.
Surely this question has two correct answers showing on the dropdown menu:
one meaning the restaurant's [singular] fish - na bialainne
and the other meaning the restaurants' [plural] fish - na mbialann.
I'm obviously having trouble with the dropdown questions! What tells us here that it should be restaurant's fish, rather than restaurants' fish? Given we have "Iasc na"... and the word is feminine?
The lack of eclipsis is what tells us that. “The restaurants’ fish” would be Iasc na mbialann.
Except this was a dropdown question... I read "Iasc na" and had to choose between mbialann and mbialainne. As far as I can tell, either would make the sentence make sense—and I didn't have the English to determine what it should be. If the lack of eclipsis is the only distinguishing feature, I'll put in an error for this type of question.
If mbialainne was one of the choices, then that choice could have been eliminated from consideration, since that isn’t a valid declension.
My prompt was a "translate into English" and it provided the correct answer as "The restaurant's (singular) fish." When I wrote "The restaurants' (plural) fish" it told me that I had a typo in my response... Clearly Duo is under the impression that it is singular.
Iasc na bialainne is "the restaurant's (singular) fish (also singular)". bialainne is the genitive singular of bialann.
Edit: "The restaurants' fish" would be Iasc na mbialann (or Eisc na mbialann).
Thank you, I read over the other comments before I posted but it wasn't super clear to me what the situation was.