It's a pity Stena don't push the route a little more.
Irish Ferries seem to be the dominant operator on the Rosslare to Wales operation. Stena have gone from Konningin Beatrix & Stena Lynx III, to Stena Europe & Stena Lynx III, now just to a rather ancient Stena Europe, which has about half the freight capacity of Isle of Inishmore.
I'm sure if Stena modernised Fishguard harbour, and brought in an efficient passenger / freighter eg Stena Nordica, they could attract more clients to the route.
P&O have operated Stena Nordica's sisters on the Larne to Cairnryan route since the 2000s, and both vessels are proving as popular - if not more than their Stena counterparts. Nordica may not be huge from a passenger perspective, but she is very efficient and would probably help secure the future of the route.
Both operators sail with mostly empty freight decks so I'm not sure that getting a vessel with more garage capacity is of great importance. Unfortunately the Wales-Rosslare market has declined from its peak and it's difficult to justify much in the way of new investment for either operator.
As for P&O's Larne ships being more popular than the Stena Superfasts, I'll just leave that one there. Although P&O have clawed a bit back recently and the freight market isn't as lopsided, the passenger traffic stats show the reality.