"Your son's dog."
Translation:Madra do mhic.
"mac" is of the first declension
confirmed by the dictionary, which also shows its male...
rules for first declension are here
so far, "mac" nom. sing. has modified to "mic" gen. sing. Now final rule: possessive pronoun "mo" causes lenition (aspiration).
The literal meaning of cuid is “part”, “share”, or “portion”. Irish uses cuid with most plural or uncountable nouns governed by a possessive adjective, so do mhac means “your son”, but do chuid mic means “your sons” (literally “your share of sons”) and do chuid bainne means “your milk” (literally “your share of milk”). In the genitive, madra do mhic means “your son’s dog” (literally “the dog of your son”), madra do chuid mac means “your sons’ dog” (literally “the dog of your share of sons”), and teocht do chuid bainne means “your milk’s temperature” (literally “the temperature of your share of milk”). It’s preferable to use cuid in these cases because it’s proper Irish.
The genitive singular of iníon is iníne. When the possessive adjective do comes before a vowel sound it is written d'.
Because the singular definite article for feminine genitive nouns is na, there is a h- prefix in na hiníne ("of the daughter"), but that's not relevant for do.