Does the rule, that says "ar + [article] -> [eclipsis] not apply in the plural?
You are correct. It only applies with the singular definite article.
The suggestions suggest that 'An bhfuil na buachaillí ocrasach?' is a valid answer. I have, however, always been told that this is incorrect. As far as I'm aware, it should be ocrach rather than ocrasach? Can anybody confirm?
Ocrach is correct, but it’s usable only as an attributive adjective; bí ocras ar should be used instead of ocrach as a predicative adjective.