Can anyone enter the lounges in the airport in Dublin, or do you have to be flying on a certain Airline?
They sell tickets to their lounges online, so it doesn’t seem to be restricted to a certain airline.
Which terminal? There are airline lounges and the airport company’s own lounge (DAA Executive Lounge). You can purchase access to the DAA lounge in T1 or T2. If you’re flying business class with Aer Lingus or Etihad you should have access to their lounges included in the ticket.