Depending on which ferry you book and in which season prices can vary a lot. If you're looking for the cheapest options then you'll have to look into the shorter crossings between the UK and Ireland. From either Holyhead, Fishguard, Liverpool and Cairnryan ferries are leaving for Ireland. From around €45,- you can book a ferry from the UK to Ireland.
The longer ferries are a bit more expensive, the ferry from Liverpool to Dublin (8 hours) is around €114 for a crossing.