Now I think I understand why we came through US Preclearance in Dublin on Sunday and still had our blue Customs Declaration form with us afterward. No one on the flight or at the airport said anything about US citizens or anyone else not having to fill in the card, so we did so as usual. Then as we approached Customs we were directed to a kiosk that scanned our passports and asked some questions, then printed out something. We then went to a Customs agent who took that printout, asked a couple of questions, and then passed us through. Now I'll know in the future.