When we booked a round trip from Dublin to Holyhead Wales on a car ferry I never realized that we would be on somewhat famous ships.
Outbound we were on the Jonathan Swift, a fast catamaran ferry that I clocked on my GPS as making 45MPH ... we made the 75mi trip in under 2 hours.
On the return we were on the Ulysses, the worlds largest ferry at the time she was launched and the subject of an hour long visit on the National Geographic Channel show "Superships".
Both trips were on time, very comfortable, and very clean accommodations/No need to spring for the "Club Class" as we did on the 3 hr Ulysses crossing -- the normal accommodations are very comfortable and the wifi is just as slow (satellite connections have lots of latency ... bad for surfing!) ... I suspect that any of them are pretty good, but we were happy with Irish Ferries