I recently travelled to Dublin with Irish ferries on the new Ulysses superferry, and the service was great.
The claim that Ulysses is the biggest ship in the world seems a bit unlikely when you are on the passenger decks, but I think that much of the space is taken up by the vast car decks, as the ferry carries many lorries.
The service is great, especially when compared with the competition - Stena Line's HSS service, which although a little faster is not very smooth (no good for those with no sea legs!), only one deck and lacks anything to keep the traveller entertained or interested.
Irish ferries also have the upper hand in that they have access to Dublin Port - right in the heart of the city, whereas Stena have to use Dun Laoghaire in the suburbs of the city. This is especially inconvenient when doing short breaks, as much time is spent getting from port to city.
They also load cars very efficiently and early onto the car decks, allowing time for you to settle down in a comfy seat on the excellent top-deck observation lounge with a huge full-width window. The ships are generally quiet, with far too much capacity, so you can catch up on a bit of sleep if you like without mcuh disturbance.
If you are in a real hurry, then they also have a lynx service - the Jonathon Swift which also uses Dublin port, and so again has the upper hand over Stena Line's high speed HSS.