"The man's son"
Translation:Mac an fhir
This is the genitive case singular; it looks the same as the nominative plural, though you can tell from the article that it's singular ("an" is always singular).
Yes, in the Genitive plural it would be "na bhfear" (the Nominative plural would be "na fir" without the eclipsis).
Here is a really handy table that shows how the article interacts with nouns depending on the first letter, gender and case: