Apparently Ryanair are less than happy with at least 4 of the routes from Shannon and news of their dropping of these routes is expected in the not too distant future. As for the viability of Shannon as a base ~ it is certainly in doubt that Shannon could support the loads that Ryanair want, even taking the "deal" that they got from SNN into account.
Ryanair have been having talks with Cork and are very keen to transfer some of the SNN aircraft to Cork and have a bigger presence there as they see that some of the routes would be supported a lot more with the greater population from Cork. They are holding out for the same kind of deal that they got in SNN. Whether they get it is another story.