Doesn't Irish Ferries run a service from dublin to holyhead, leaves around 9pm at night, allows for turnaround in holyhead, then back in Dublin by approx 6am following morning? Would that be what you're looking for?
I know Stena lines used to run a dedicated party/booze cruise from rosslare to fishguard, where they'd have an allnight party, with dj etc, but I think it's only certain dates, every 6 months or so, no harm to ring both Rosslare harbour and Dublin port and check what the schedules are.
Flax - many many moons ago I used to work on that particular route, and that's a regular occurence in winter months, not for the faint hearted I can tell you
I have memories of being stuck off the coast of Wexford with shelves of alcohol tippling over in the shop, and plates and cups flying off the shelves in the galleys, with passengers on their knees puking for Ireland all along the corridors. Lovely stuff!