The Irish ferries are a completely different thing, aren't they. There is at least some competition there, and that probably makes a difference. In recent years I have used those ferries more often than the IOMSPC, and they are generally trouble-free. The ships are modern, clean, spacious, with facilities for adults ad childre, and staff who treat you like a customer, rather than a nuisance. Of course there are sometimes problems, but that's acceptable. the dictatorial attitude of the IOMSPC, where you cannot even find out what exactly you are buying (defiite date and sailing time) or how much it will cost is appalling.
Imagine going into M&S for a ready dinner. When you choose it you have to pay a deposit...when you get to the till they tell you the full price, and if you don't like it they say "Tough...we keep your deposit!" No, I don't think so. M&S know how to treat a customer.