The International Transport Workers Federation has accused Irish Ferries of denying employment rights to its staff by using a so-called "flag of convenience" on its MV Normandy vessel.
The company has registered the vessel in the Bahamas, which means it does not have to abide by Irish legislation on employment rights.
ITWF spokesman Tony Ayton said: "[Staff on the ferry] will now be subject to whatever laws are in the Bahamas and I regret to have to say that there's practically no law in the Bahamas to protect workers."
SIPTU members at Irish Ferries went on strike for 10 days earlier this month in protest at the company's plans to "outsource" jobs on the MV Normandy to cheaper foreign workers.
The company claimed the cost-cutting measure was necessary to help it compete with rival ferry firms and low-cost airlines.