Passengers arriving Dublin in J are entitled to lounge access for same day onward connection on same ticket. Lounge staff hold a printed passenger list but the J boarding card is all you really need.

Passengers departing LHR to DUB to a J connection are entitled to lounge access in LHR and DUB.

AerClub Platinum or better flying EI to LHR and then BA onwards regardless of class of travel get lounge access at LHR T3/5