By Peter Moor.
P&O Ferries has resumed its sailings between Larne and Cairnryan, with the ship being staffed by agency workers.
A union boss says some of the crew are previous P&O employees who were sacked and then applied for agency roles.
The company came under fire after sacking 800 workers around the UK with no notice last month, before replacing them with agency labour.
P&O Ferries is operating services from Northern Ireland to Scotland at 4pm and 23:59pm on Sunday 10 April.
The company is using its European Causeway vessel on the route, after the ship had been detained for a time over safety concerns.
However, she passed an inspection on Friday, allowing the firm to resume a limited service on the route.
Micky Smyth, a representative for the union Nautilus International says that the ship is now being staffed by agency workers.
"The unfortunate thing is everybody onboard that vessel from the Captain down is an agency worker. There is no permanent employees employed by P&O Ferries" Mr Smyth said.
Mr Smyth claimed a number of people onboard the vessel used to be employed by P&O Ferries but are now agency workers.
He said: "We believe that a few have returned because obviously, words don't put food on the table and there's no jobs out there for maritime professionals .
WATCH: Micky Smyth from Nautilus International speaks to UTV:
"Our message is that our campaign will continue against P&O Ferries until such time that they come round the negotiating table and consult with the maritime unions, namely Nautilus International.
Mr Smyth says he was "absolutely stunned" when around 800 workers were sacked from the ferry company on 17th March.
"I worked for the company previously. I left it eight years ago. I served this company like everybody else, loyally for 27 years.
"There's people that have been here for almost 40 years," Mr Smyth added, saying that he believed P&O Ferries had treated it staff outrageously.
UTV News has contacted P&O Ferries for comment.