To get to Belfast by car, take the M1 from Dublin or the A1 from Newry. For those coming from the west, take the M2 from Derry/Londonderry.
If travelling by rail, there are regular services from Dublin to Belfast on the Enterprise train. There are also trains from Bangor, Coleraine, and Derry/Londonderry.
If you are in the nearby town of Lisburn, you can take the train or bus to Belfast. If you're visiting the Giant's Causeway, consider taking a scenic rail journey to Belfast afterwards.