As many as 10,000 holidaymakers face having their summer ferry sailing plans cancelled.
A new ship being built in a German shipyard is not likely to be ready for delivery to Irish Ferries in time to honour bookings for sailings in July.
The new vessel, the WB Yeats, will not be available to take passengers and vehicles on sailings booked between July 12 and July 29.
All booked sailings between those dates have now been cancelled by Irish Ferries.
In a statement, the company said it had been in contact with people with bookings between those days.
A spokesman for Irish Ferries said the delayed delivery of the ship was a setback for the company but it was hoped a large number of the affected passengers would be accommodated on sailings from Rosslare aboard the Oscar Wilde.
The company has 'blocked off' space on the Oscar Wilde for affected passengers.
If spaces are not available then Irish Ferries will offer a full refund.
Passengers affected by the cancellations were contacted by email on Friday and offered a €150 voucher as a goodwill gesture from Irish Ferries for use on future sailings. The company said the eventual delivery date of the completed ship had not yet been fully confirmed.
Irish Ferries decided to cancel all those July dates in recent days "in the interests of minimising the level of potential disruption to our customers," said a spokesman.
In the official statement, the company's managing director, Andrew Sheen, said: "We have just been informed by the German shipyard building the WB Yeats that her delivery to Irish Ferries is likely to be delayed.
"We are currently in contact with affected customers and offering spaces that we have held on the MV Oscar Wilde that are close to their booked sailing date.
"In the absence of an acceptable alternative, customers will, of course, be entitled to a full refund."
Sailing times that affected passengers choose to be rerouted on might be a day earlier or a day later than intended as there was no exact overlap between the scheduled departures of the WB Yeats and the Oscar Wilde.
The new €147m ferry was due to start servicing routes from Ireland to France and the UK in July.
It will be the biggest vessel to shuttle passengers to and from Irish ports.