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.
