Well they could go on strike. I think an airline this short on crew is going to be pretty powerless if half the workforce don't turn up one morning, unionised or not.
But of course this won't happen, the majority of Ryanair pilots either have no backbone or would use such an event as an excuse to boost their own career prospects ("look at me, I turned up when everyone else didn't").
Excellent news nonetheless, and certainly more cancellations to follow.