Marked correct while using incorrect eclipsis, lenition and plural spelling.
I recommend Duolingo strictly enforce the correct use of eclipsis, lenition and plural spellings. I have got these incorrect several times and Duolingo continues the lesson only citing an incorrect spelling. I believe this would be helpful for the learner.
I'm one who was keen for it to be more strict on the fada:-) I'm not sure I feel the same about this:-) (Although I will admit to feeling a brief annoyance at getting it right some of the time, when I had missed/included some of these elements.) It's obviously something that's important, but I suspect that if it were more strict, it might push the task further out of the 'fun' area, and more into the 'battle' zone:-) I'm already having to repeat lessons more often than I recall doing in German (my other 'from scratch' language on DL), in order to complete them satisfactorily. That being said, I still feel my grasp on eclipsis and lenition in particular is fairly tenuous! Maybe if I had to be more accurate, the extra practice would have them fixed more fully in my brain:-)
This is a tricky area to deal with. In this course we teach the traditional rules for eclipsis and lenition used in standard Irish. In reality, however, initial mutation is much more complicated and variable. For example, "on the ground" is written in Irish as ar an talamh - but if you are using one of the alternative sets of rules for lenition and eclipsis, it would be correct to write this as ar an dtalamh. Similarly, ag an bhfear can be written as ag an fhear if you are using yet another set of rules. All of these systems are included within the current edition of the Official Standard of Irish - but you should not "mix and match" these rules. As long as you stick to a single set of rules, what you write will be acceptable.
The two options are: (1) strictly accept only the traditional standard rules that we formally teach in the course, and mark other valid answers wrong - or (2) accept all grammatical answers, which runs the risk that learners may end up being marked correct when in reality they made a mistake.
I appreciate the difficulty with Gaeilge Dún na nGall having a different rule when it comes to what follows "ar an" etc. But in general, I think you should find a way to mark it as wrong when I've effectively given something the wrong gender. If I use "le" instead of "la" in French, I'm marked down; the same should be true in Irish if I leave out a séimhiú in the Tuiseal Ainmneach.