We used to go to Plunketts but we were students and still visited til recently when it had gone downhill. Not particularly romantic tho but was 'our' place
Depends what you want mate really. Hotel du vin as suggested by Paul D is meant to be pretty special but haven't been. Personally I like El Gaucho - Argentinian steak house. Ask is an incredible venue (The Assembly Rooms), but not the best for food. Best italians for food are ToTo's and Strada, athough La Vecchia Scuola is an incredible building with views of the minster. Loch Fyne is a seafood restaurant that was recently refurbished and the building is pretty impressive. beyond that you have your real expensive places like The Blue Bicycle.
Plunketts was tex mex wi best blue black homemade burgers & ribs this side of the Atlantic but as I said got pretty prettied last time we went and quality not as good