You can't go to Coruña and leave without going to this popular bar which is an institution among…read morethe locals. It's located in the main eating-out area of Coruña so there is always the hustle and bustle of good atmosphere. It is very cheap, all tapas are 1€. I especially recommend the croqueta de jamon (ham croquette) which is massive and filled with visible chunks of ham (unlike most croquettes).
Come here before lunch as an aperitivo or at night for a few tapas. Be warned though, it is always busy and you will most likely have to eat standing which is why I prefer to come here for a few and then move on to the next place.