The Dublin castle basicaly doesn't exist anymore, it was destroyed during the uprisings and rebuilt in a diferent style than the original. If you epect a real castle you WILL be disapointed, especially since it is now surrounded by office buildings. What is worth visiting are the castle grounds, maybe bring your Starbucks in and simply relax there.
Unique Suggestions: Skip the castle part and visit the castle grounds. If you want to go into the castle check with the tourist office if there are any interesting exhibitions during the time you're in Dublin.
Fun Alternatives: This is not the only castle in Dublin area - take a trip to Malahide, visit the castle and then have a walk on the Marina.
This view of the castle grounds, was really more interesting than the castle itself. I was disappointed in the castle itself because due to previous destruction of the original castle during uprisings and wars and what not, when it was rebuilt, it was in the fashion of the current trend in decorating, we would call it "colonial" here in the U.S. This took place in the 1700’s and the place looked like George Washington had once lived there, not any Irish nobility. It was well cared for and our guide through the castle did a fine job of telling of it's history. I was just expecting something else.
The Dublin Castle is a bit of a con as there is hardly any of it left. There is one side of wall and the rest has been modernised and there are hardly any historic artefacts to see. It is free for most of it unless there is an exhibition on.