Damage to the thruster was discovered and the unit was removed in January when the Irish Ferries-owned vessel was inspected by the yard's engineers during a routine docking.

As soon as the unit had been repaired by the manufacturer the vessel was brought back to Belfast for its re-installation.

David McVeigh, of Harland and Wolff, said while the job was small it was significant in that the vessel could only afford to be taken out of service for a very short period.

"Time is critical to passenger and commercial vehicle ferry operators and we are in a position to reduce the amount of time needed to complete essential work," he explained.

The Isle of Inishmore can sail at cruising speeds of more than 21 knots and the thrusters are used to help steer the 185-metre long ship when docking.

It can accommodate 856 cars and 2,200 passengers.

Belfast-based Harland and Wolff Heavy Industries' main dock is supported by the twin 800-tonne gantry cranes Samson and Goliath and large hard standing lay-down areas.

The company focuses on four main areas of activity. These are: ship-building, repair and conversion; design engineering; the design, manufacture and assembly of renewable energy hardware for wind, current and wave energies; and steel fabrication and installation.