Twonk:
SAZ9961:
Another cheap ferry to consider is the Stena Fleetwood-Larne crossing. Like the Liverpool ferry, its a truck ferry with space for cars, and can be a bit slow. There are overnight crossings on thursday nights. Like the Liverpool ferries, a comfy cabin, dinner and breakfast are all included in overnight crossings.
What do you call cheap? I looked on their web site and although you can only book as far ahead as January a return crossing for car & driver plus passenger is £328.
Check again; January return for car and driver from £170 return, £25 each way for an extra passenger. You might add maybe £30-40 to that for a spring crossing (plus whatever for inflation). A night sailing is £240 plus your passenger.
Norse Merchant are £170 for the 5 day excursion, on a day saling, £295 on a night sailing
Irish Sea ferries are never going to be Calais-booze cruise cheap. Cheapest regular ferries will be from Stanraer, but if you are coming from the South, you may have a significant cost in fuel. Swings and roundabouts. Many years in the Province has given me experience of virtually every ferry crossing, and what constitutes "cheap". Cheap is £10 or less each way (pay a bit more if you get an overnighter with a cabin).
Another budget ferry is Liverpool-Dublin Port by P&O.
http://www.poirishsea.com/passenger/Dublin_Liverpool.htm
Another truck ferry, again best get this overnight. This truck ferry is (or was) the least geared up to deal with car drivers. I took it once from Dublin to England. You check in to a smoke filled truckers office, looking circa 1950. Cars are last on, and park on a ramp. Me being in the smallest car, was loaded last. I had to reverse off at the over end, 100foot into pitch darkness. It uses Liverpool Freeport, so you spend some time whizzing around a container yard, trying to find your way out. But it is cheap, with grub included.