Irish Ferries says services on its Dublin-Holyhead and Rosslare-Pembroke routes have been cancelled because of what it said was unofficial industrial action taken by SIPTU Ships Officers.
Meanwhile, SIPTU has claimed that ships' officers on Irish Ferries ships were told to stop loading passengers and freight with immediate effect.
Irish Ferries said the ships' officers had refused to obey an order to depart from UK ports at the normal scheduled time - and that they were now in breach of their contracts.
Two ships, the Isle of Inishmore and the Ulysses - are tied up in Pembroke and Holyhead with their crews aboard.
A spokesperson for Irish Ferries said the ships are likely to remain where they are for the time being.
A skeleton crew will be left on board each ship for safety reasons. All other crew-members will be repatriated as soon as possible.
The ships' officers were due to commence an indefinite strike on all Irish Ferries routes from 6am tomorrow morning in a dispute over its plans to contract out 150 jobs.
It had been expected that ferries services would operate as normal until then.
