Outside this season bargains will be found easier and not all companies are fully booked months in advance. The mild climate and fairly stable weather of Ireland are well-known, so why not take a cruise outside the main season?
Why not indeed - you will have more peace and quiet, less waiting at locks, no noisy neighbors at the marina. On the other hand you may have to heat the boat, pushing energy costs up. And not all supply stations may be open. Daylight will be shorter in supply, fogs are more likely and even some ice may form. Also bear in mind that you will more than likely get wet and cold occasionally. If this does not detract from your idea of a good time, go for it!