Most ships do not have irons or ironing boards, they are a fire hazard. Some ships do have self serve laundry, Princess and CArnival do and there may be irons and boards in those laundry rooms. Best bet is to pack all your dresses, suits, etc. in the plastic cleaner bags. I've yet to have to iron any of that, it really works. Just about all the ship today have a laundry service, they will pick up and deliver it back to you, just ask your cabin stewart and he'll take care of it.
bj, you will find that most of the ships have irons in their self-serve laundry. They are permanantly attached to the wall to make sure you don't take them to your cabin. They are a fire hazard and crew members really don't like them for this reason. We take a steamer with us that works on almost everything. If it is something that we just have to have pressed then we would just use the ships laundry service as they are very reasonable.
Not all cruise lines have self service laundries though. For example, Celebrity doesn't/ So if you don't know and have no access to a deck plan, just ask here and someone will be happy to tell you if you need to bring a steamer.