Irish ferries:
"Pets are allowed to accompany passengers travelling with a car. Your pet must be taken on board the ship in your car and must remain either in your vehicle or in the kennels provided by Irish Ferries for the duration of the voyage. Your pet is not allowed on the passenger decks at any time.
The kennels are situated on the car deck, can be booked in advance and are free of charge.
If your pet is staying in your car for the duration of the journey there is no need to declare him/her before travel".
P&O:
"Can I take my pet with me on P&O Irish Sea ferries?
Yes, pets will be permitted to travel in their owner's vehicles, and must remain in the vehicle for the duration of the crossing, on payment of the appropriate fare. Bookings including pets must be made in advance of travel. Pets (excluding guide dogs) will not be allowed in the passenger areas of the ship. Access to the car deck during the crossing is not permitted other than in exceptional circumstances, at the discretion of and accompanied by a member of staff".
Stena:
"There are no restrictions on taking your pets to Ireland or Britain. Cats and dogs may travel in your car free of charge on the Irish Sea. For health and safety reasons your pet is not allowed onto the passenger decks (with the exception of guide dogs) and must remain in your vehicle."
Chris