Ever more soaring oil prices are blamed for Stena Line's decision, which increases the usual crossing time between Dún Laoghaire in Co. Dublin and Holyhead in northern Wales by 16 minutes (to an average of now 115 minutes).
Journeys between Larne in Co. Antrim and Stranraer in Scotland are extended by 14 minutes (to an average of now 119 minutes) as well to cut costs.
"In the current circumstances most passengers realise that just as they are making some difficult choices, ourselves who operate in the transport sector have to do that as well," a Stena Line spokesman said.
"Compared to the conventional ferries it is still a lot faster. We provide a quality of service and people come back because of that quality."
Stena Line recently opened a new terminal in Belfast, which will shorten the distance to be covered, although the slower speed means that it will now have little impact on crossing times.