Unfortunately, there aren't shuttle packages available on weekdays. You could however get the bus from Europa Bus Centre in Belfast City Center. The 238, 538, and 238b all run hourly to Banbridge. From there you could take the walk to the Game of Thrones Studio Tour which is just under 30 minutes or could take a taxi once you get off the bus. Have a look here
to see your options.
Let me know if you have any other questions and I would be happy to help.