At least 60 employees who lost their jobs when P&O Ferries sacked them on Thursday are from Ireland, their trade union has stated.
Maritime union, Nautilus International, official Mickey Smyth said he estimated that 25 workers from the Republic and 35 from the North are among the 800 who were dismissed without notice on Thursday.
Most work on the European Causeway vessel which sails between Larne and Cairnryan.
The workers affected were employed by a hiring agency in Jersey and are subject to UK law.
Mr Smyth said the workers on the P&O Ferries ferry between Liverpool and Dublin are not affected as they are employed under Dutch law.
The company said its services between Liverpool and Dublin resumed on Saturday
The Dublin-Liverpool route is serviced by two roll-on, roll-off passenger and freight ships, the Bermuda-flagged Norbay and the Dutch-flagged Norbank.
The Norbank left Dublin Port at 7.30am on Saturday morning for a scheduled sailing and is due back from Liverpool on Saturday.
“The Dutch obviously have different employment laws and they are protected. It is not part of the dispute. The other vessels are chartered vessels which are impacted,” he said.
Mr Smyth said he hoped the Irish Government would intervene to persuade P&O Ferries to change its mind.
The British government has said it is unhappy with the decision and is investigating the legalities of it.
Mr Smyth added that he felt the actions of P&O Ferries copied the example of Irish Ferries which sacked 543 workers and replaced them with cheaper foreign workers.
P&O Ferries terminated the employment by conference call on Thursday. They are being replaced by cheaper agency staff.
The decision has caused outrage among unions and politicians in the UK and there have been calls for the boycott of the ferry company.
P&O Ferries, which transports passengers and freight, operates four routes: Dover to Calais; Hull to Rotterdam; Liverpool to Dublin; and Cairnryan to Larne.
It has 2,200 employees remaining in the UK and began operating ferries in the 1960s.
In a letter to staff Mr Hebblethwaite said P&O Ferries was losing £100 million a year and had to change its business model.
The Norbank is crewed by Dutch officers and Filipino crew.
Nautilus general secretary Mark Dickinson expressed the union's anger at this move by the company, and warned there was a clear threat to public safety posed by agency crew.
“Clearly, we are outraged by this suggestion. We call on the public to boycott P&O Ferries and to not travel with this shameful business,” he said.
“We ask the government to act in support of loyal UK seafarers, and we demand to know how its vessels can run with agency crew who are unfamiliar with the vessel.”
His union tweeted: "P&O chief executive Peter Hebblethwaite reckons the Liverpool-Dublin route will restart today. Nautilus demands to know how the vessels can run with agency crew unfamiliar with the ship? The government must urgently inspect the vessel and make its report public."
Liverpool West Derby MP Ian Byrne tweeted: "I'm sure the good folk of Liverpool and Dublin will boycott this scab vessel if it restarts today. A huge test for this government as well, deeds not words please."
