try www.poirishsea.com
i went over in the car 6 weeks ago on the day sailing.
£112 return.the downfall is leaving liverpool at 9.30 in the morning and not getting to dublin until 5or 6 in the evening.its very boring but the food is provided as well both breakfast and dinner,not great but better than paying for the crap on the holyheads boats.
they have a nightboat as well but its expensive when you include the mandatory cabin.
its a freight boat and may not have a lift if you got anyone whos not too good on their pins.
i presume you know how to get to lisburn,out of the dock and follow the belfast signs.