Irish Ferries is being accused of using foreign crews to pay below the minimum wage.
SIPTU said the company's vessels are all registered abroad to avoid Irish wage laws.
The Union will hold a protest later at Dublin Port later to highlight the issue.
Irish Ferries said their vessels have been registered outside of Ireland since 2006 and its activities comply with the law.
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Jerry Brennan from SIPTU said the company is Irish in name only.
He said: "If they did fly the Irish flag they would have to pay national Irish minimum wage.
"So they circumvented this by using manning agencies located in Cyprus, they fly the Cypriot flag on the ship and they get away with not paying national minimum wage."
