I have never had a bag weighed by Aer Lingus, but have never travelled with them since they started charging for hold baggage last year. What route are you flying? If it is DUB - UK you can use the check in machines, however if it is DUB - ORD you will have to use a check in desk. A way around this is to check your bag into left luggage while you check in, the retrieve it, as long as it meets the size for security you'll have no problems.