The Reichstag. One of my favorite travel memories took place here in November, 2003. Nowadays you have to register in advance to visit this historic building. I've made my registration and even reserved a time for dinner at the top.
this one! great bath house for relaxing. it's outdoors and super hot and lots of chillin. try out the different pools, different temperatures and one of them has a jet-propelled merry-go-round (that makes it sound a bit cooler than it may be)
I would recommend you come to the Blue Lagoon directly from the airport if your flight arrives at 7,8,9 in the morning. Hotel Check-in is not until 3pm and this place will relax your mind a body from the over night flight.
some of the best entertainment and traditional irish music is in the pubs around dublin! walk around at night and go into all the different pubs and you will see some of the best irish musician "jams" you've ever seen! they can get crowded, but wiggle your way in anyway! :) its worth it!