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.
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.