If you get to the gate you are unlikely to have a problem since the SNN services are rarely full. The issue may be getting it past the Aer Lingus staff at LHR T2 check-in, since baggage charging is a key aspect of EI's business model. Now I don't do this checked baggage malarkey at all, so I'm not the best person to comment on the likely reaction there, but there may be something in the Aer Lingus forum if you search.

For the return, DUB-LHR often is a full service with luggage issues, so if you are taking 2 bags it wouldn't be very surprising if one is taken at the gate. If the flight isn't full I doubt there would be a problem.