Ardaghman,
As you are connecting onto an AA flight, you will have to claim any checked in baggage and clear customs with it in ORD, then drop it off at the AA interline desk. Since you have to do this anyway, I woud suggest using online check in / check in kiosks and picking up your onward boarding card at the AA desk in ORD. That way you lessen your chances of EI check in staff asking to weigh your handluggage. In my experience, if you were not checking in ANY luggage they would be more likely to ask to weigh your handbaggage.

Please let us know how u get on and your experience on the EI flight. It will be crewed by a SNN base crew and there are suggestions that these are less friendly than the other bases.