I Love travelling from Rosslare to Pembroke on the Inishmore BUT you don't see as much ships as going to Holyhead from Dublin!
When you travel from Dublin to Holyhead at 08:05 on Ulysses, you might see a cruise ship heading into Dublin (during summer), Stena Adventurer following behind you, you also pass the H.S.S. Stena Explorer, Stena Nordica, Epsilon and then the Swift passes out the Ulysses.
Where as when you go to Pembroke, you only see the Stena Europe following the Inishmore for a certain time before she swings off to Fishguard and you MIGHT see some cargo/container/tanker ships travelling up the Irish Sea to maybe Belfast, Dublin or Liverpool, etc; from Continental Europe or somewhere else.
I know when I last travelled on the Inishmore back in June, When we left Rosslare, the Stena Europe was following us for about an hour and a half, then she swong off to Fishguard. One of those big car carriers went across the Inishmore's bow at one stage but thats it, you might see one or two tanker ships docked in the Haven.