On conventional ferries
If you are travelling on conventional ferry (not a fast ferry or catamaran) there is normally space available in the kennels and you should also be able to have your dog on all deck areas (but not inside) with you. Strictly speaking, this at the discretion of the ferry company but in general you should have no issues - though please take a muzzle with you just in case and keep your dog on a leash at all times. Small sized dogs and cats can travel in the ship as long as they stay inside a special travel box.
The other option, for absolute peace of mind, is to book a pet cabin. These are like normal cabins but have linoleum flooring. They are also available for day crossings at reasonable prices. This way you don't have to worry about finding space in the kennels and you and your dog or cat can relax. In July and August ferries can get very busy with people and other dogs.
