"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 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.