and Rosie was allowed to go on the passenger deck with me but I was not allowed to take her anywhere where food was prepared.
That actually shouldn't have been allowed on Stena; you must have had someone who was kindly towards dogs and didn't think others would complain! I've taken that ferry and the formal rules are they have to stay in kennels or in the car. But once when I went walk-on with dogs in a crate (roll on crate) from Holyhead to Dun Laoghaire, the staff let me sit with them in a stairwell rather then put them in a kennel, and didn't mind if I took them out and held them.
I have taken my dogs many times on the ferry with no issues at all. Always check with the company as some require you to let them know in advance that you are travelling with a dog. One that goes to Scotland did and another did not. Irish Ferries to Holyhead from Dublin does not.
Normally you simply keep your dog crated in the car. There is very little of the car decks that is exposed to the outside -- if it looks as if you will be positioned into such an area, just ask the guys directing the cars to let you hold back and park in the enclosed part of the deck. Or book to put your dog in the on-board car deck level kennels.
Generally you should not feed a dog on the morning of travel (in a car or a ferry or plane...). Many will just get sick from the motion or the anxiety of being left, if they are not used to car travel in a crate (it's a good idea to get a dog used to this in advance and it is the safest way to drive a dog around anyway).
I give water just before we board then again as soon as we arrive. I put a light sheet cover over the crates so the dogs cannot see out and are cozy and enclosed and leave the car locked with windows cracked open slightly for some air.
I just was over and back on irish ferries with all four dogs to Wales and had a great time, dogs are always very relaxed on the drive and the ferry.
You cannot take a dog out of the car and bring it anywhere else on board BTW they must either be safely in the car in a crate/cage or placed in the pre-booked kennels. In all the times I've gone I've always heard dogs on the decks in cars and never seen anyone use the kennels. Follow best airline practice when choosing a good crate -- dog should be able to fully stand, turn around and lie down.
Enjoy your trip!