I bought a 'bridal cover' from the Internet - it was big enough to take two ball gowns and was not expensive. I carried it over my arm as hand luggage and the dresses didn't need any attention when I unpacked in the cabin.
I put mine in a gents suit carrier which loosely folded in the middle and the 2 handles clipped together so it just looked like a holdall/overnight bag - they were ok when I took them out.
I put mine in my case lat and layer tehm with tissue or plythene. The majority of them come out fine or look fine after hanging up for a while in the cabin Only had one time when one looked very creased. Just called the cabin steward and he arranged for it to be ironed and delivered back to my cabin in a few hours. They did a very good job too.
Leave them in the plastic covering which dry cleaners use and fold them, if possible, as close to where the bodice section joins the skirt or lower section. Always make sure that they are the last items to go in the case. When you get on board have a really hot shower and then hang the in the steamy bathroom being careful to keep them away from any wetness or where they might fall into wet patches. Jeeves says that you should roll them up to travel but I've never had the courage to try this... Id.
If packing evening dresses in a suitcase I always enclose them in polythene. Either the kind you receive covering clothes from the dry cleaners or I buy large sheets of fine plastic sold to protect your carpets and furniture when you are decorating and are available at DIY stores. This has always worked for me even on a South American cruise with long flight and 3 day tour before embarking the ship.
Just to let you know that there are 'self service' Laundry rooms on board . You can avail of the steam irons free of charge as aslo the washer and driers. However, I would advise that you take with you a gents hanky or similar so as not to iron directly on your beutiful dresses. Do avoid leaveing this little excerise until the last minute as you may well find you will have an audience of all those who are also waiting their turn too. Have a wonderful hol.