Friday, departing 07:00 or 21:10
Saturday, departing 07:00
£41.97 tax inc'l
(Other flight times available, at considerably higher cost)
P&O Irish Sea
£20 day sailing, £25 night sailing - 8h sailing time
Isle of Man Steam Packet Co
£25, £2 additional for reserved seating - 3h, 45m sailing time
Norse Merchant Ferries
Unknown - Site says "under construction" for route information
Hope that helps! The Isle of Man Steam Packet Co seems best, since the sailing time is half the other two, for only an additional 5 quid. Best to check for yourself, of course.
I only checked one-way...