"The man's son"
Translation:Mac an fhir
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:
Fear is broad and m., hence in the 1st declension. So gs. is a palatized version of ns., slender fir.
The definite article triggers lenition of m. nouns in the gs. -> f(h)ir.
declension of the definite article:
gs. m. an, f. na
Clearly, it's gs. here, matching fhir.