UK citizens don't require a passport to enter the Republic of Ireland. However, you'll need some form of ID for the ferry operator.
This is from the Irish Ferries 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".
This is from the Stena Line website:
"British citizens travelling to Northern or the Republic of Ireland do not need a passport but are advised to take a form of identity"
Chris