Passengers who had their ferry bookings to France cancelled because delivery of a new ship was delayed can apply for compensation, the Irish National Transport Authority (NTA) has ruled.
Irish Ferries had maintained customers had been given adequate notice and there would be no compensation.
It said it was "disappointed" and would appeal the decision in the courts.
Irish Ferries said the cancellations were due to a delay in delivery of its new ship, the WB Yeats.
The NTA received about 100 complaints in relation to the cancellations.
If Irish Ferries fails to comply with the notices issued by the NTA "it commits an offence and is liable on summary conviction, to a fine of €5,000, or on conviction on indictment to a fine of up to €250,000", a statement issued by the authority said.
However, if a passenger accepted reimbursement of their ticket price in full then they are not entitled to any further compensation.
Irish Ferries said the cancellations were due to "extraordinary circumstances which were completely outside of the company's control".
"Since the delay was due to unforeseen delays by the shipbuilder FSG, and was notified to passengers months ahead of planned sailings, Irish Ferries does not agree that the company infringed the relevant EU Regulation," a spokesperson said.
"In dealing with its customers Irish Ferries believes it took every reasonable action to provide passengers with alternative travel options, from a no-quibble immediate refund to allow them to make alternative travel plans, as well as alternative sailings on the Oscar Wilde out of Rosslare Europort and land bridge alternatives via the UK.
"Irish Ferries has, on numerous occasions, attempted to engage with the NTA by offering to enter into a mediation process without any preconditions.
"The NTA has not taken up this offer. Irish Ferries will appeal the NTA's decision in the courts (including, if needed, the European Court of Justice)."
Jenny Redman of the Consumer Council in Northern Ireland told the BBC's On Your Behalf that passengers should have a record of their sailing and expenses if they wish to apply for compensation, including their original tickets and details of any additional mileage travelled because of the cancellation.
"Our advice is to contact Irish Ferries again," she said.