You can find plenty of information at the Official Belfast Tourism site where you can download free city guides, check out the hottest new restaurants, and figure out where you want to stay.  Guides are available in English, French, German, Spanish, Italian, and Dutch currently.  Here, you will also be able to print out a reasonably detailed map of the city in addition to walking tours around town and a guide to historic pubs so that you can get going the moment you get off the plane.

Once in Belfast make sure to drop in to the Belfast Welcome Centre, 47 Donegall Place, Belfast. Tel: 028 9024 6609  Email: welcomecentre@belfastvisitor.com (located in the heart of the city centre and main shopping area) for free guides and maps as well as booking tours and events,  a great range of gifts available in the shop and a bureau de change.

You may also want to check out the Northern Ireland Tourist Board website if you plan on visiting other places while you're in Northern Ireland.