I don't think pre-clearence is a huge impediment to operations at Cork. The reality is that the runway is challenging, the market relatively untested and there are two established airports with service to the East Cost within ~2 hours drive. The US relines are clearly happy with the performance of their SNN operations, or they would have pulled out. Giving up a known market for an untested one, even with a shed-load of marketing support is a tough sell. If Cork manages to keep a high profile with US travel journalists, or managed to get a large HBO/Netflix-style production then things could change and act as an attraction to aid the support of an East Cost route. Dubrovnik, New Zealand and Iceland are two locations where this has happened recently.