P&O and Stena January offers
Both P&O and Stena Line are currently offering up to 20% off ferry crossings. P&O's offer is only available on the Larne to Cairnryan route, and you need to use a promotional code on the offer page at http://www.poferries.com/en/troon-larne/offers/try-sailing-from-cairnryan?promotion=EBO20. They have just extended their offer to bookings made up until the 29th of February, and it is for crossings made up until the 15th of December 2016. Terms and Conditions can also be found at the above link.
Stena Line are also currently offering 20% off the price of a crossing as part of their January sale (theres also up to 20% off items purchased onboard), however both the Cairnryan and Liverpool routes are included in this promotion. Again you need to use a discount code, and bookings must be made by the 27th of January for travel up until 16th December. The code (and terms and conditions) can be found at http://www.stenaline.co.uk/sale .
As always please check the terms and conditions on the operator website.