I've never noticed their sales to make much of a difference. But I do know, as I book a lot of flights with them, that if I book for say April next year it's entirely possible they will change the flight times. They generally do their schedules in November and April in my experience. If the new flight time was inconvenient they used to let you change to a more convenient day, but they are much stricter now.
You'll only know this later, which is why you should be flexible. Are your dates the Easter school holidays (and I don't just mean Irish school holidays). If yes and you end up going a day earlier because holiday rates are 'generally' based on Sat to Sat, then you might have an accommodation issue on one night. And if Ryanair pull the route (unlikely) make sure your accommodation can be cancelled, up to say a month in advance.