Hi Sunaholic
I stayed in the Clontarf Castle Hotel when I stayed in Dublin. It is a bit pricier as it is 4 star, but well worth it for the fact that you are staying in a castle. For an anniversary or special occassion it would be perfect.
There are 2 bars, the Temple Bar and the Drawbridge, where you actually walk over the drawbridge thru the huge door. The restaurant is wonderful too and has the authentic windows, wrought iron lights and you eat off pewter plates and drink out of pewter wine goblets!
The hotel is wonderful to explore and has hidden doors and tapestries on the walls.
It is about 2 miles out of the town but we used to walk along the Bay after breakfast (which we needed to do!) and then hop on the bus the rest of the way. It didn't cost much and there were plenty of buses and the walk was pleasant by the Bay and the park.
Anyway, take a look if you can and see what you think.