Returning to the US out of Dublin, Ireland, we found that we had to go through two layers of "US Pre-Clearance" security in Dublin airport. First was the usual removal of shoes, putting all belongings on X-ray conveyor belt, walking through metal detector. Next was passport control, which entailed laying your passport on a scanner, having your photo taken, and taking the resulting printout to an agent who showed you a photo of your checked luggage and asked you to verify if it was yours.
This all takes extra time in the airport. Luckily we had allowed more than 2 hours from our connecting fight, as it took well over an hour to wait in the lines and get through. By the time we came out, our flight to the US was listed as "FINAL CALL." If our Dublin-bound flight had been late, we might have missed our connection.
Some years ago I flew to the US from Toronto, Canada, and encountered a similar process there. I didn't remember having to do this from anywhere in Europe, but I'd never flown out of Dublin before. I heard on the news today that this system is only in place at a few airports -- among them Dublin and, you guessed it, Toronto -- but the TSA plans to implement it at additional busy airports around the world, such as LHR.
I suppose it is a security enhancement since it's intended to prevent people with questionable credentials from even entering the US in the first place, instead of apprehending them once they land at their destination US airport. For law-abiding travelers, it is just something to be aware of when planning how long you'll need to get through security on your US-bound flight.
BTW, I highly recommend Aer Lingus. It was one of the cheapest fares we were able to find, and we had no complaints about the quality of service and condition of the airplanes.