No. Ferries are hard to hijack - to the best of my knowledge it's only been done once - and it's a fairly pointless exercise anyway. ("Take me to Cuba, Captain" won't get you very far!). Similarly, a bomb on board might do some damage but it's not going to have the same effect as on board a plane. Ferries are no higher a security risk than trains, so you'll hardly see any security (although, of course, there will be a few low key people keeping an eye on things).
You don't need a passport to travel to the Irish Republic (assuming that you're a UK citizen). However, Irish Ferries have this on their website:
"Irish Ferries recommend all passengers bring a passport with them. Irish and British citizens do not strictly require a passport to travel between the two countries - some form of identification is however required.
Photo ID, Bank Cards or bills for house utilities addressed to you are all acceptable and useful means of identification".
There are no customs formalities when travelling between any two EU member states, whether by plane, train, car or ferry.
Chris