Carnival has laundry rooms that offer coin operated washers and dryers along with irons and ironing boards.
You are prohibited from bringing irons on board because they are a fire hazard, especially when used in your cabin. One of the worst things that can occur at sea is a fire and the cruiselines work very hard to try to prevent it.
You are permitted to bring a clothes steamer though.
I love this amenity on Carnival and it allows us to pack lighter because I don't have to bring so much. I can do laundry on the ship half way through our trip and cut the amount of clothes we bring in half.