We have exported sheep to NI - pretty much as gavo has said. What we did find was that the people who were importing insisted that there were no rules and we just needed to get on the boat! Not so, and fortunately we didn't listen to them. Animal Health will be able to guide you, plus your vet for all the blood testing, health certificates, paperwork etc. You cannot take animals on the fast catamaran ferry, but you can bring the empty trailer back on it - cuts the time on the sea by half. There are no checks on leaving Britain, but you will be pulled over and inspected when you arrive at Larne, all of which takes up a big chunk of your journey time, which for us was limited to 8 hours because we don't have the long journey certificate. We got there in the nick of time. It was interesting to do once, but now we have the T-shirt we don't want to do it again