The reliable money is made from consultants and contractors who fly every week to projects they are working. They make up a bit over 5% of passengers, but 15%-20% of revenues and sustain the route for 6 months a year.
I couldn't see Shannon-Amsterdam poaching many of them, although there is the potential to grow similar business from/to the Mid-West.
The next group who contribute good money are connecting business passengers. These are often late bookings, so the revenues from the leg to Amsterdam can be pretty good. The fact that Cork has no long-haul pushes a lot of these through Amsterdam (although far more go through Heathrow).
Again that group would be hard to tap. Many are going to the US, so if they were to go to Shannon, they would be more likely to get on a westbound aircraft than fly to Amsterdam. Then again, Ryanair don't offer connections anyway.
The place where Ryanair could really poach passengers from Cork-Amsterdam is from tourists visiting South-Western Ireland (mostly Dutch, but also from Belgium and north-western Germany). This group makes up a substantial portion of summertime travelers and they do often pay substantial fares. However, it is a group that could equally easily be enticed to leave from Eindhoven.