A P&O ferry travelling between Cairnryan and Larne was left adrift off the Co Antrim coast.
The European Causeway was cut off five miles off the coast of Larne, according to tracking website Marine Traffic.
The vessel is able to carry up to 410 passengers, but it has not been revealed how many are onboard the ship.
The website stated the vessel’s automatic identification system status is set to “not under command” which is reserved for use when a vessel is “unable to manoeuvre as required by these rules and is therefore unable to keep out of the way of another vessel”.
A spokesperson for the RNLI said that three lifeboats had been sent to the scene.
Gareth Fuller
The vessel is said to have been hit by a 'mechanical issue.'
Gareth Fuller
An RNLI spokesperson said three lifeboats had been sent to the scene.
P&O said that the European Causeway had been affected by a “mechanical issue”.
The company tweeted: “Due to a mechanical issue with the Causeway in the Irish Sea, tugs from Larne and Belfast were deployed to guide it back to port.
“Once the ship is back in Port a full inspection will take place".
A spokesperson for P&O Ferries said the European Causeway was now travelling to Larne “under its own propulsion”.
The spokesperson said: “Following a temporary mechanical issue, the European Causeway is now continuing on its scheduled journey to the Port of Larne under its own propulsion, with local tugs on standby, where it will discharge its passengers and cargo as planned.
“There are no reported injuries onboard and all the relevant authorities have been informed.
“Once in dock, a full independent investigation will be undertaken."
The Rail, Maritime and Transport workers’ Union (RMT) said the reports were “deeply concerning, not least for the agency crew and passengers on board”.
The European Causeway had been detained at Larne after an initial inspection by the Maritime and Coastguard Agency (MCA) on March 25 uncovered 31 safety failings.
This was due to safety concerns after the company sacked nearly 800 seafarers and replaced them with cheaper agency workers.
The ship was cleared to resume serving the Larne-Cairnryan route a fortnight later following another examination".