Is there any rule for determining which nouns have weak plurals, or does one just have to memorise?
If it's a feminine noun ending in a broad consonant or a masculine noun not ending in -ín or -ir it might have a weak plural.
That's all you can really say, Irish has a crazy plural system.
Actually, I read somewhere that weak plurals ate "almost exclusive" to the 1st and 2nd declensions. I took that to mean, conversely, that most nouns in these declensions have weak plurals. If you want to check, I believe the source was the awesome nualeargais site...
"The dog's food." Does the placement of that apostrophe not mean this is "The dog is food."? It must not because this skill is littered with similar instances.
It's a possessive apostrophe.
The Irish phrase is a genitive construction, and it could also be accurately be translated as "the food of the dog".
The English to Irish exercise "The dog's food" is indeed ambiguous, but because the possessive meaning is so much more obvious, I doubt if is bia é an madra has been added as an acceptable answer.
Yes it would not seem likely. And thank you sir for the lesson in basic English. It's a wonder I manage at all!!
The genitive of singular masculine nouns after “an” is lenited. C.f. cat an fhir. So this rule applies to at least masculine nouns of the first (fear) and the forth declension (madra).