An Irish Ferries stand-off is in its fourth day, with union claims that 30 crew members are barricaded in ships at Pembroke Dock and Holyhead.
The Isle of Inishmore has been in Pembroke Dock since Thursday
They are protesting at plans to replace staff with cheaper workers from abroad.
The dispute is set to escalate after port workers said they would not handle a ferry arriving from France on Sunday.
Taoiseach Bertie Ahern criticised Irish Ferries' handling of the row, but the company said it had to bring in foreign staff in order to remain competitive.
The MV Normandy left Cherbourg at 2330 GMT on Saturday and was due to dock in Rosslare at 1645 GMT on Sunday.
But port workers in Rosslare, and their counterparts in Dublin, announced they would turn the vessel away when it arrived. There were reports suggesting that the Normandy could head to Cork instead.
They were the latest developments in a row which began on Thursday when some crew members seized control of the Isle of Inishmore after new eastern European crew members boarded.
The Isle of Inishmore had been due to run between Pembroke Dock and Rosslare. Another vessel, the Ulysses, which was due to go from Holyhead to Dublin, also remained in dock.
Irish Ferries said the workers were there to familiarise themselves with the vessel and their roles and the security personnel were put there to ensure the continued access of company staff and port officials.
Norrie McVicar, of the International Transport Federation, complained because officials of the National Union of Marine, Aviation and Shipping Transport Officers (Numast) were not allowed on board at Pembroke Dock. Some crew had asked for union help.
Mr McVicar said about 15 crew were barricaded on each ferry in Holyhead and Pembroke Dock.
The firm has told passengers the dispute may go on until Monday
Unions said the existing staff were upset by the manner of the security guards' appearance.
They said the men had boarded in Ireland as passengers, but had then changed into their uniforms in the toilets as the vessel approached Wales.
Irish Taoiseach Bertie Ahern described the company's approach as "a retrograde step".
"This is not in line with Irish industrial relations, they are trying to turn back the clock," said Mr Ahern.
"About everything that they have done in their handling of this I fundamentally and totally disagree with."
Irish Ferries has said it had been completely open about what it was doing over bringing in new crews as part of a cost-cutting exercise. It said it was justified in employing security staff.
Spokesman Alf McGrath said: "The security measures were necessary because in December of last year Siptu (Services, Industrial, Professional and Technical Union) staged two strikes... and totally locked up the ship in Holyhead and would not allow regulatory agencies or any management on to the ship.
"So we have a duty and responsibility to protect our assets."
Bob Carrick, general secretary of the Seaman's Union of Ireland, said up to 90% of the crew had agreed to voluntary redundancy terms which had allowed the firm to bring in new workers.
But Mr Carrick said the wishes of the remainder to keep their jobs and terms of conditions should be respected.
He said: "That's not a choice, if somebody says you can keep your job but we're going to slash your wages in half."
Around 70 crew members are thought to be on board the Isle of Inishmore. Police said they were keeping a minimal presence at the port in Pembroke.
Irish Ferries has told passengers it does not expect services to resume until Monday.
A spokesman said there were "no plans for formal talks between the management and workforce, but managers are represented on both ferries and so there is the possibility of dialogue".