Northern Ireland Tourist Board
Tip from Derek:
Giant's Causeway is really a day trip from Belfast. Typically one would go up the Antrim Coast Road and enjoy the scenery en route to possibly visit the Carrick a Rede rope bridge and the Giant's Causeway. Then it's back via Portrush (and the Amusements) and Portstuart for some Morellis' ice cream. After that, head back to Belfast as fast as you can via the motorways.
I would be very surprised if any Coach company (other than the 'get you from A to B' type) would treat Giant's Causeway as less than a day. I could be very wrong as it is some time since I have lived in NI.
If time is so short, perhaps you might like to consider Carrickfergus and Carrickfergus Castle instead? The Mount Stewart estate just outside Newtownards on the Ards Peninsula is very nice too.