Translation:You eat rice and you drink milk.
In English "you eat rice and drink milk" is ok; would "Itheann sibh ris agus olann bainne" be acceptable? Or does every verb need a subject?
I'm not certain that the ordering that you suggest is incorrect, but "Itheann agus ólann sibh ..." is fine, and avoids repeating the subject. When the objects are different as here, I'm not sure if it can be abbreviated though.
Doesn't Hiberno-english (the type of English spoken in Ireland, just like American English is spoken in the US) sometimes have "ye" as the plural of "you"? Should "Ye eat rice" be accepted?
Yes, "ye" is the most common 'you plural' found in Hiberno-English, along with a few other variants. You can report it if you like, but I'm not sure if they'll start accepting it. I've already reported one instance of "sibh = ye", but they haven't gotten back to me about it yet (they've been exceptionally quick in general, however, I must say. Each morning I wake up to a dozen e-mails of new translations accepted that they've approved).
But anyway, I too would like if they accepted "ye". In French and Spanish it doesn't bother me as much, but because when I learnt Irish in School it was "ye", I find myself saying it and typing it without thinking. But we'll have to wait and see what they decide.
When translating for Italian, I use "you all" for the plural "you" in order to separate the plural "you" from the singular "you" (especially in my head), so I'm doing the same here and it seems to work so far. :D
is this similar to a vocative case, in that it's addressed to a person or group? Or is it just a plural "you"?
No. Irish and English both have separate forms for the simple present ("you eat"/itheann tú) and the continuous/progressive present ("you are eating"/tá tú ag ithe).
While some other European languages don't make this distinction, these tenses are not interchangeable in Irish or in English.
No. Itheann sibh is addressed to two or more people. Itheann tú is addressed to just one person.
Is anyone else getting the correction "Ye eat rice" etc? I get that it's a plural/singular thing, but that's still not the right answer.
When you submit a wrong answer, Duolingo does a crude alphabetic match between your wrong answer and each of the "acceptable alternative answers", and offers the one that is closest to your wrong answer, as if to say "look, you nearly got it right!". Unfortunately, because the contributors were very generous in accepting a range of dialectically diverse suggestions for "acceptable alternative answers", learners often encounter suggested corrections in both Irish and English that, while not strictly wrong, are unhelpful.