You don't "check in" at customs unless you have something to declare anywhere I have been to including the US.

Also, this article is about about passport control not customs.

I thought there would be someone to check my passport. The whole airport was empty. You hop off the pane and you are in Ireland. It was like driving to another US State, you just go.

I thought that was odd, going to other countries back then, someone would always stop me and check my passport.