I have to fight this instinct too!
I've only had it accepted once, to answer "An bhfuil bróga glasa uaibh?", (where if I remember right it's compulsory to answer 'ye' and I'd finally got into the habit of 'you', ...sigh), and no I don't thanks. Blue suede only please, but I don't want to answer for the rest of ye.
"you have coats" just means that you possess coats - the coats could be in a bag, or hanging on the backs of your chairs. In fact, ask a large enough group of kids, and one of them is bound to answer "I have a coat, but I left it at home".
Possession is indicated with the preposition ag - Tá cótaí agaibh is "you have coats".
oraibh- "on you" indicates that the coats are being worn. The sentence can also be translated as "there are coats on you", but "you have coats on" is a more likely expression in ordinary English speech.