Your question is a bit confusing. In the default nominative case mac is singular, mic is plural.
In a genitive construction like teach do mhic, mic is singular. But unless you are clearly using it in a genitive construction, mic is plural.
So I think your question should really be "how do I tell if I'm supposed to be using the genitive?"
If you don't know the difference between nominative and genitive, then you obviously won't realize that "How do I tell if mhic is plural or singular?" is the wrong question, but it's the wrong question because you'd also have to ask "When is "fhir" singular or plural?", and so on for a whole category of nouns where the plural nominative and the singular genitive share the same form.
While using a search engine to translate English into Irish is not a good strategy, asking a search engine a question about grammar terms like "genitive case in Irish" is usually helpful. There are a number of Wikipedia articles that explain it, but the first hit when I did that search is from Bitesize Irish. It's always worth reading a couple of different articles in a case like this, because different writers might make more sense to different people.
The n-prefix in n-iníonacha is just that, a prefix, and it doesn't have a standalone existence. The d' in d'iníonacha isn't a prefix, and it already has a pronunciation. While the two vowels can merge, that won't cause the consonant to change, because if it did, you wouldn't be saying "your daughters" any more.