Initially the crew tried to tackle the fire themselves, but they quickly realised they needed to call for help.
Local fire crews and specialist teams from Belfast worked at the scene, along with the Port Authority and the ship’s crew, to bring the incident under control.
A spokesperson for Northern Ireland Fire and Rescue said all crew members were accounted for and no-one was injured.
Station commander Kieran Doherty explained: “In this instance the cabin was sealed, the door was locked and there was smoke detection in the room which is linked to the bridge to the captain and that’s where the captain was alerted.
“We’ve determined the cause as an electrical fire. Something similar to your normal house fire where you’ve got electrics plugged in.
“We’re certain that it was a wall heater with an electrical fault, it’s caught on fire, caught the sofa, caught the bed.”
Operations at the port returned to normal within minutes of the fire being put out and on board the Marit, the clean-up is under way.