You will need to clear Canadian customs on arrival at Dorval. You usually have to retrieve your checked luggage at this point unless the agent that checked you in has the correct tags for transit luggage in Canada (not too likely in Europe I have found). Check what they have done otherwise you may be standing by the wrong carousel. You also have to clear US customs at Dorval - this is done by going to gate 99. You also have to retrieve your luggage at this gate and take it through US customs with you - you can choose whether to take it from Canadian Customs to US customs yourself or to check it in, in which case it will appear on the carousel in gate 99 at the back. After clearing US customs you deposit your baggage on a belt.
Sorry have not been on AC in Y class.