Workers employed by Irish Ferries are to take industrial action from next Thursday, in a row over outsourcing of employment.
Notice of the as yet unspecified action has been served on the company by the marine officers' section of SIPTU.
It said the action would affect all of the company's ships, but the precise manner in which it would be implemented would be decided closer to next Thursday.
Mr Alf McGrath, the company's director of human resources, said it could respond by suspending services.
The row is over a decision by the company to outsource employment on its service between Rosslare and France.
SIPTU accused the company of breaching a Labour Court agreement on the need to implement cost savings by agreement.
Mr McGrath, however, said the court was dealing with a separate issue.