Why is only "ag" acceptable and not "ar"? Is "ar" just more commonly used for "on" (ar an mbord)?
Ar is more generally “on”, but among its many uses is a temporal “at” (e.g. ar a dó a chlog). Ar scoil for “at school” is an exception rather than a rule.
Potafocal doesn't have much to say about attending "marriages" (pósadh), but for "wedding" (bainis) there are examples of both ar bainis and ag bainis.
Yes, I’d imagine that ar bainis is closer to ar scoil in the “participation/membership in” sense, which tends to be an “at” sense in English (at least in the US English locative sense).
Given that Duolingo seems to be using pósadh where bainis might be more appropriate, it should probably accept ar an bpósadh while it's at it.......
THANK YOU, I was getting confused between pósadh and bainis and now you've articulated why.
Thanks for everyone's replies. It's getting so I'm developing an instinct for some of this stuff, but I also know nowhere near enough to trust that instinct much of the time!
Where does the b come from in the beginning of the word?
Many of the prepositions, like ag, ar, le, etc, cause eclipsis after an, and words that start with p are eclipsed with b.
So Ag pósadh is "At a wedding" but Ag an bpósadh is "At the wedding".