£5 foot passenger (each way) on either the Cairnryan-Belfast or Liverpool-Belfast routes.
Book by 19th March (this weekend only!) to travel anytime until 15th December 2017.
Cheap sailing if you don't need a cabin (at additional cost).
A bit more than the current £2 holyhead-dublin or fishguard-rosslare deals, but on a much more popular route.