The Carnival Dream sails out of Port Canaveral, Fla. and sails on Saturdays, alternating routes between Eastern Carb. and Western.Carb. This particular Sat., the 27th, it was scheduled for the Eastern Route, with calls at Nassua, St. Thomas and St.Martin. It has to go south, irregardless of which route it sails and therefore will be sailing away for the storm--however, the storm has already passed Port Canaveral and as you won't sail until approx. 4 p.m tomorrow ( SAt. ) evening, I would think you will get little effects from the storm. The only thing is that it's possible that Nassau has been damaged so they may not be able to stop there but so far, there's been little in the way of reports of damages from the Bahamas.
Do as you feel like you must but were it me, I would lay the worry about Irene on the shelf and forget it.