I find the grammatical structure very puzzling. Thank you, Mod, for posting the link below to the notes.
Would it be incorrect to say 'Tha doileag aig am mac agam'?
I think 'an nighean agam' was used earlier in the lesson for 'my daughter'.
Or is there a convention like with 'my husband'
So "tha mo mhac doileag agad" would be incorrect?
It would be incorrect. It would look like an ungrammatical attempt at saying something like you have my doll-son (you have because that’s what tha X agad means; but then mac doileag doesn’t really make sense anyway).