In reply to Flinticus:
We took two dogs to Ireland (Holyhead - Dublin) last year. Our vet advised that a dog passport was required - and this was not cheap when the cost of updated inoculations was added in, about £160 per dog!
In the event, we were not asked to show the dog passport at either ferry terminal and could have got away without one. However, I imagine that you would be refused passage on the boat if they did decide to check and that could also be a very expensive little number.
Personally, if I was doing it again and could afford the inconvenience of having to reschedule my trip to Ireland, then I think I would pay the little extra for a ferry ticket that was fully refundable/transferable and would chance bringing the dogs without passports.