Would I be correct to say "Mo mhairteoil agus a ull", instead would be, "my beef and HIS apple" ?
Yes, that's correct.
thank you, slowly getting to grips with "his and hers" now
Since “beef” is a mass noun, it would be better to say Mo chuid mairteola … rather than Mo mhairteoil … .
How do you say, our bread?
"Ár n-arán" would be my suggestion.
"An aran siad" I believe.
Why is it 'húll' and not 'úll'?
Because a úll means “his apple”.
Sorry it means her apple and I was just being blind. -.-
Why is it not just "Mo mhairteoil agus a úll"?
As jay1916 and scilling have already pointed out in the other comments, a úll means "his apple".
We're supposed to eclipse after the possessive adjective a so why not n-húll? I know that's wrong but I'm seeking to understand.
Only plural possessive adjectives cause eclipsis - a n-úll means "their apple".