I'm an old hand at ferry crossings now. I've gone on Irish Ferries umpteen times and on Brittany ferries less often. Brittany is much nicer- nice food, nice staff, clean and new. Comparing them is like comparing a hostel to a hotel. Both will do the same job, but one is just nicer. Brittany of course is much nearer to me, but as we mainly travel off-season, we usually have no choice except to go on Irish Ferries.
Sue Ellen is right about the cabin- the higher up you go, the better, and you CAN choose which floor you want, whether you want a window or not etc. I would never go without a cabin, and nowadays always go for a high cabin with window.
If you go on Irish Ferries, bring your own food, the food is appalling and expensive. If you go on Brittany Ferries theres no need, the food is great and good value.
Both of them offer entertainment- cinema, depending on the season there may well be a cabaret act in the main bar. Fun for all the family.
If I had a choice I'd go with Brittany Ferries, but not having that option wouldnt put me off Irish Ferries, as long as I had a cabin and my own food.