Saw this on the Irish Ferries website:
http://www.irishferries.com/ie-en/news/News140617/
First paragraph suggests that you can book Irish Ferries Rail and Sail tickets through Pembroke, which I don't think is correct - only Stena services come up if you try to book a ticket through to Rosslare.
I see that several stations can be booked directly to and from Ireland.
You can even get a quote for a one way ticket from Ireland (in sterling) but regrettably only gives UK stations as possible machines you can pick the tickets up from.