If you want to stay in a Castle but not at the price of an arm and a leg try Clontarf Castle -
My fiance and I stayed there a few months ago and it was great. Nice sized rooms and it still had the conforts of a hotel but a little different as it was a castle.
It wasn't far from the centre. There is a shuttle but we took taxis as they were cheap as it was not far from town.
They have a nice bar and everything.
Try it out.
I have looked into this
I have looked into this awhile back, I am part Irish and one day want to visit the area. These are afew places I have found in my travel searches: Fitzpatrick Castle Hotel Dublin & The Clontarf Castle Hotel, both are breath taking! Also if I can get the chance I was to visit these as well: Swords Castle (this was built around 1200), Howth Castle (15th century, not open to public but you can visit the grounds) & Malahide Castle (did you know that one family keep control of the castle for 791 years? I think that is tradition if I ever saw it!). Good luck with what you pick. I think it is a personal choice, though many of the castles listed by myself and others are nice choices, in the end its all about your vacation!