Eastern Europeans employed on a ferry that operates between Dublin and the United Kingdom (UK) are being paid just over €2 an hour, according to union officials.
An inspector with the International Transport Workers' Federation (ITF) yesterday said some of the 22-strong crew on board the vessel also claimed they were being treated like "prisoners" and had not been paid for more than four months.
The boat is a roll-on-roll-off freight ferry that is part of the Norfolk Line fleet and sails regularly between Dublin and Heysham port, near Morecambe.
It is registered in the Bahamas and is Latvian-owned.
Norfolk Line yesterday said it was "surprised and concerned" to learn of the allegations.
In a statement it said while it did not have responsibility for the crewing or technical management of the vessel, it took the issues extremely seriously.
"Norfolk Line has a strong relationship with the ITF and recognises the importance of the work they undertake.
" We have urgently discussed the matter with the ITF and will meet with their inspectors on Monday in Dublin," the statement said.
The charter agreement between Norfolk Line and the owner of the vessel says the ferry must be operated in accordance with ITF pay guidelines.
The company said it is seeking clarification from the vessel's owner concerning this matter.
Ken Fleming of the International Transport Workers' Federation said that he was invited on board the vessel by a number of crew who claimed that double book-keeping was being used and that they had not been paid any wages for several months.
There were 22 crew members on board, most of whom were of Polish, Ukrainian and Russian origin.
The ferry is not obliged to pay the Irish minimum wage. However, the international rate as agreed by the workers' federation is around $3 (€2.30 an hour). Agreed overtime rates increase to between $10 and $16.50 an hour.
The transport workers' federation said there was no evidence to suggest these rates were being paid.
Mr Fleming said he was in the process of considering the "lawful arrest" of the vessel on behalf of the crew members to reclaim wages in accordance with their employment contracts.
The vessel sailed back to Heysham yesterday afternoon and was scheduled to sail back into Dublin Port early this morning.
"Siptu and the Rail, Maritime and Transport Union in the UK will be calling for the fullest investigation by the Irish and UK maritime authorities on all Irish ferry and roll-on-roll-off vessels trading in the Irish Sea.
" The evidence will also be submitted to the European Commission," Mr Fleming said.
Siptu general president Jack O'Connor called on authorities to enforce international law and rectify the injustice the seafarers had suffered.
According to the ITF, an able seaman should be earning about $3,200 a month.
These rates were evident in the first contract of employment provided to some employees on board the vessel, according to the union.
However, a second contract of employment indicated that workers were receiving about $1,000 for 365-plus hours, or just over €2 an hour, the ITF said.
