Translation:In Ireland, barmbrack is eaten on Hallowe'en.
at Hallowe'en is rejected even though it seems more natural to me.
In the USA, "on Halloween" is much more common — almost universal. As a side benefit of learning some Irish, I enjoy learning about these sorts of variations in English! [Edited to add the following...] That's not to say that Duolingo should reject "at Halloween," since that's so common on one side of the Atlantic.]
Dinneen’s dictionary agreed with you, defining bairghean breac (the older spelling of bairín breac ) as
“barnbrack,” or “barmbrack,” the currant cake used on Hallow Eve, etc.
The EID only has an entry for “barmbrack”, and the FGB only gives the spelling “barmbrack” in its entry for bairín. Neither spelling is currently present in the NEID.
For those of us for whom this is a new thing: http://en.wikipedia.org/wiki/Barmbrack
And, a recipe (under the colcannon one): http://www.irishcentral.com/roots/traditional-irish-recipes-for-barnbrack-and-colcannon-this-halloween-106286868-237785401.html
DL needs a look at common nitpicky points. One is the contrast between (largely) US usage on and (largely) British usage at
US: on Christmas, on Thanksgiving, on Easter, on Hallowe'en Br: at Christmas, at Thanksgiving, at Easter, at Hallowe'en.
It is irritating to have the perfectly natural They eat barmbrack in Ireland at Hallowe'en rejected. Even other sequencing (In Ireland etc) is rejected if one retains at. Infuriating.
Do you say "on your birthday" or "at your birthday"?
The use of "on" for an event is hardly a US usage (and "at Christmas", "at Thanksgiving" and "at Easter" are perfectly normal constructions in the US) - your problem is that you have grown used to thinking of Halloween as a "season", rather than an event, but the oíche in Oíche Samhna makes ar the more natural preposition, just as it is perfectly natural to say "on Christmas Day" and "on Easter Sunday" even in British English.
The key point is that, unless it is absolutely unreasonable (ar scoil), your best bet is to stick with the literal translation of the preposition - translate ag as "at", ar as "on" and sa as "in". The purpose of Irish to English exercises isn't to demonstrate your ability to create perfect idiomatic English, it's to demonstrate that you understand idiomatic Irish. And just as it's reasonable to say "we always had fireworks and a bonfire on Guy Fawkes", it's reasonable to say barmbrack is eaten on Hallowe'en.
(That still leaves open the question of whether you should ever translate "at Halloween" as ag Oíche Shamna, but that's a discussion for another day).
The problem with your advice is that there is not a single literal translation of Irish prepositions to English. "Ar" doesn't only mean "on" nor does "ag" only mean "at". Irish prepositions are quite fluid and what they mean varies as to the context in which they are used.
Ar can mean any of "on", "in", "at", "within", "of", "by", "for", "to", "with".
- ar talamh = on earth
- ar neamh = in heaven
- ar mo ghualainn = at my shoulder
- ar amharc = within sight
- ar do chosúlacht = of your appearance
- céim ar chéim = step by step
- ar mhaithe leis féin = for his own sake
- ar deis = to the right
- ar mhórán saothair = with great effort
In relation to time there is:
- ar maidin = in the morning
- ar a sé a chlog = at six o'clock
For a fuller account of the meanings of ar see here
I didn't say that you should always translate ar as on. I said that "unless it is absolutely unreasonable (ar scoil), your best bet is to stick with the literal translation of the preposition" - in other words where "on" is reasonable, and you wouldn't be completely thrown by seeing "on" in the English sentence, leave it as "on". Even people who don't say "we wear costumes on Halloween" probably wouldn't quibble if they came across that phrase in a blog post or a magazine article, so in that case, it isn't necessary to change it to "at". On the other hand, if you read "I walk on school every day" you'd know that something was wrong, - it's not reasonable to translate ar as "on" in that situation.
You seem to be missing the point about the literal translation. You did say that unless it is absolutely unreasonable it is best to stick with the literal translation of the preposition and you go on to indicate that the literal translation of ar is "on". But this is not so, there is not one single literal translation of ar as I have shown above. We may have been taught at school when starting to learn Irish that ar meant "on" but that was only to keep things simple for beginners.
I did a steaw poll on this one amongst my friends and acquaintaces. Interestingky, the younger ones slightly favoured "on" but the older ones predominantly "at". That led to a discussion of " at or in a city" which, interstingly, showed that mt older friends prefer at Oxford or at Brighton but in London or in Paris. It seems size matters
I take yoyr poinr about seasons, but I can't see me ever saying on Christmas. However, if Irish says Ar Oíche Shamhna then that is what's critical here. GRMMA.
I wouldn't normally say "on Christmas" either, but I would always say "on Christmas Day", and the "Day" could sometimes be dropped. Growing up, Halloween was much more focused on the night itself, so "on Halloween" doesn't seem wrong to me - "we dressed up on Halloween", "we bobbed for apples on Halloween", "our neighbor always had fireworks on Halloween" (the "night" is assumed in those examples) but of course when it came to brack, which would have been available in the shops for a week or two before and after the day itself, I would say "we ate brack at Halloween"!