Mourne Seafood Bar on Bank Street is the best place in Belfast to eat fresh fish and shellfish. The menu changes daily based on what comes in from the boats, but Strangford Lough oysters, Mourne lobster, and Kilkeel crab are reliably available. The cooking is straightforward — fish this good doesn't need much — and the prices are reasonable.
The basement oyster bar is the most atmospheric part of the restaurant, a low-ceilinged room where you can eat a dozen oysters and a glass of Muscadet for a very modest sum. The restaurant gets busy for dinner; the early evening is easier. The fish and chip lunch (cod or haddock in beer batter, proper chips) is one of the best in the city.