The Ferry companies aren't as fussy as some airlines.
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"
So Irish Ferries don't even require you to have photographic ID. A household bill will do.
P&O are a bit vaguer. They state:
"Photgraphic identification must be carried by all passengers and shown when required when travelling on the Dublin - Liverpool route."
Given that P&O don't specify any particular type of photo ID, an 18+ card would almost certainly be acceptable.
Stena simply state this:
"British citizens travelling to Northern or the Republic of Ireland do not need a passport but are advised to take a form of identity".
Once again, there is no specific requirement for photo ID, so almost anything should do.
In practice, I'd be amazed if anyone actually asks to see any form of ID (unless, of course, you're collecting your tickets at the port). An 18+ card would almost certainly be acceptable in the unlikely event of someone doing so. If you're still in doubt, phone the ferry company to check.
Chris