Yes, in theory you can access T3 so long as you stay airside. You may be challenged at T3 security in that the boarding pass says T2, but if you show lounge access rights then that should let you through. If you then go on to T2 you have another run via T2 security because your passage between T3 and T2 means mixing with arriving passengers again. There is a pay as you go lounge in T2 (Plaza Premium), and the Aer Lingus lounge is available for Advantage and AerSpace fares.