Ryanair seem to operate:
Within the next week or so: VERY expensive
1-3 months: Cheapest fares
>3 months: VERY expensive
Obviously it is not as black and white as this but they do seem to pick and choose their lowest fares as and when they choose.
This is completely opposite to U2 who seem to work very much to the original plan of first come first serve. I guess that is something businessmen/women as into...