Translation:The queen died and the public were sad.
According to the American Heritage Dictionary of the English Language (4th ed.), in American usage a collective noun takes a singular verb when it refers to the collection considered as a whole, so that the normal translation of this sentence in the US would be "the public was sad". Nothing said about usage in other parts of the English-speaking world. To my ear, if one wanted to use a plural construction here one would say "the people were sad". This is not accepted as a correct translation here, though. Would that specific phrasing be represented by a different expression in Irish?
Bhí brón ar X would still be used as the expression, but the article + noun X would depend upon which sense of “the people” was meant (e.g. na daoine, an dream, na géillsinigh).
I believe Americans are more likely to use a singular verb with a collective noun and the British more likely to use a plural one, whether or not the public or team or jury is/are acting as a unit. Compare American and British sportscasters and the frequency of "team is" vs. "team are." (Duolingo accepts both "public were" and "public was" for this exercise.)