I've eaten in the Clarion more than once, food has always been good and the service excellent on most occasions but slightly lacking on one (it's amazing how one bad one can make you forget about the numerous good ones). Overall I'd definatly say it's worth a try, I would (correction.. I will) certainly go back.
The Opera House isn't far from the City Centre, so anywhere in Cork City would be a managable walk away from the OH. There are other threads on restaurants in Cork if you'd like to investigate fully, but the suggestion of Aroma is a good one! (only eaten there once but food and service were both outstanding)