Valid on all cruise and fast ferry services, the deal applies to new reservations made before April 11 and at least 48 hours in advance of travel.
Recently, the Company unveiled plans to increase its sailing capacity on the Irish sea through a €315m investment in two new vessels.
The 54,985 tonne W.B Yeats is due to enter year-round service later this year, followed by a 67,300 tonne vessel scheduled for delivery before Mid-2020 and set to be the largest cruise ferry in the world in terms of vehicle capacity.