Pre Customs
Well there was mention in one press report of up to 50 Corporate Jets a day using the facility. This assumes that they won't want to carry on to the US and clear customs at an airport there.
It'll only work if airlines want to use airports in America that don't have customs clearance. The savings on custom clearance is far less than on pre immigration. Even then the time saved in US has to be made up by turning up earlier at SNN or DUB.
The fact that other european countries haven't bothered to request similar facilities from the U.S. speaks for itself.