When I got to the Dublin airport, I was surprised that there was some odd security and something called “US Preclearance” to go through before heading to my gate. The departures screen in the main part of the airport listed not the gate but the preclearance directive, and upon arriving at the preclearance area I had to give my customs declaration as if arriving in the US. Another odd twist was that while passing through passport control, they showed me a picture on the screen of my luggage and asked me if it was mine, which it was. It was a little surprising to me that they were (even) that sophisticated, and after I declared that it was in fact my bag, the agent took my customs declaration and gave me back my passport. At first I was a little pissed at this whole procedure (figuring I was going to have to go through the same thing upon arrival in the US), but the agent explained to me that having passed that point, I was now just like any other domestic passenger inside the US (despite being in Dublin), and there would be no other passport control. This made me very happy, as this is one of the biggest pains in the ass upon re-entering the US.