I see that there are two ferries from the UK to Dublin: Stena Line and Irish Line Ferries. Irish by far the fastest, but a little more expensive. Stena is almost double the travel time and a few dollars cheaper. Any ideas?
this may help you decide between the two ferries from Holyhead to Dublin
http://www.seat61.com/Ireland.htm#complete%20timetable
In addition, P&O sail from Liverpool to Dublin but do not take foot passengers.