If no one has welcomed you to Cruise Chat yet, let me be the first!
There are a couple of ways to do this. The easiest is a car rental. Many of the major rental companies have drop off locations near Port Canveral and provide shuttle buses to and from the port. So you can rent a car at the Orlando airport, use it while at Disney, drop it off in PC before your cruise and then rent another car for the return to Disney. The car rental shuttle make periodic stops at the Port so you can just grab one on the return leg and get to the rental car place. By the way, I have done this but only did Disney on one leg of the trip.
Other options are transportation companies. If you google on limo/shuttle services in orlando, you will find a couple of options. The expense I can't speak to, but that truly depends on the number of people you have in your party and how long you will be in Disney before and after your cruise (how many days you have a rental)
Then the other way would to book Disney shuttles from the airport and back for the per cruise Disney trip and then instead of getting on the plane, rent the car at the airport and go to PC so you don't have to pay for the rental while at Disney.
And another thought, take a Disney shuttle to Disney, call a rental car company to have you picked up and brought to a rental agency near Disney (I'm sure they are there) and then you can return the car at PC and not pay for the days at Disney. Unless you really need to have a car while at Disney, it seems to be a waste to have it sitting there. But you would have to quote it out each way and see if it is cheaper to keep the car than to pay for transport to and from the airport to Disney.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007