Hi,
best ferry is irishferries.com (ulysses cruise ferry) - be careful about booking the swift one because when weather is bad they cancel the sailing and irish sea is very unpredictable. alternatively stenaline.com - depends what part of dublin u are staying because there is two ports. holyhead to dublin city (central dublin) or dun laoghaire. u need to keep checking fares but usually the earlier u book the cheaper. they advertise from £59 each way, never got one that price but then again i always book last minute which can b expensive.
ireland is getting very expensive because of the euro so recommend u book nice B & B's. u can also drive about then when u see somewhere u like u can always just get somewhere on the spot. may shouldnt b that busy but wrap up well as can be a bit chilly.
hope this helps a bit and happy holidays