The National Gallery of Ireland is well worth a visit if you're into that sort of thing; it's just off the NW corner of Merrion Square and v. handy for Trinity College and the Book of Kells. It also has a nice cafeteria-style restaurant in a glass-roofed courtyard where they do a decent lunch. The National Museum, Natural History Museum Irish Parliament Building and St. Stephen's Green are just around the corner, and all the above are within easy walking distance of Pearse station on the DART rapid-transit line.
Dublin is fairly pricey these days because of the dreadful Stirling/Euro exchange rate.
The King Sitric is wonderful. It's on the DART so if you over-indulge you can stagger on the train back to Belfast. The food and service are excellent and it's not unknown for seals to surface just offshore and look hopefully at you and your fish dish.
Can't remember but I think they can also organise direct sales of smoked salmon for you to take away (much better price than at Dublin Airport). If not, they can certainly tell you where to go in Howth, to buy some.
My mouth is watering just thinking about it.....enjoy (you lucky so and so) :-)