Varies in Dublin. If you miss the three on the 17:00 - 20:00 slot of a Saturday you can do two on the 01:00 - 04:00 slot (Sun) for the same reward i.e. priority until 23:59 Monday. Bit annoying that you can't benefit from both but, hey ho, that's the way it is.
It's imperative to have priority in Cork because with only one peak time job required fer priority almost every taxi in the city has priority on a constant basis and if you don't have it then you are well back of the field.
What I find annoying is the Urgent Pre-books offering instant priority, that is of no use if you already have priority as your priority time is not extended if you take the urgent pre-book.