Ryanair already have a vast network from Europe and the U.K into Dublin. It'd be far easier for them to build a transatlantic hub from there rather than from Shannon or Cork. I still think Cork's best chance of a viable transatlantic service is with Aerlingus. Yes it would dilute their Shannon yield but they would gain passengers from United and Delta and also pick up those that travel with BA and others through LHR.
I'm not saying it's likely to happen but if I was a bookmaker I'd make them firm favourites to be first out of the traps if a service ever gets going.