According to a web search I have just made, there is also still a Pembroke to Rosslare service, so I think this deserves a London train too!
I used to cross to Rosslare quite often - from both Pembroke and Fishguard (to the extent some of the crew recognised me)
By rail, Pembroke Dock station in spite of its name is a walk of around a 2 km walk to the ferry dock, and it's an hour and 20 minutes by train from Carmarthen to Pembroke Dock station. Fishguard Harbour station is alongside the ship, and rail time from Carmarthen is 52 minutes.