EI Immigration Checks...
Our arrival in LAX was punctual (early, but the taxiing, waiting, etc. ate up the time we “saved”,) and just before Immigration, we were asked if we’d arrived on Aer Lingus and passed through without any doc checks (even as pax from other airlines were arriving and having their photos taken, etc.) I’d have thought there’d be more than an “honor system” to assure people arriving were on the pre-cleared flight!
I've always found this interesting as well - in the busy days when EI was flying to Newark, I took this route often and found the honor system to be in place. I also noticed some restrooms just before immigration that someone could have snuck into on an earlier flight and then blended into the EI passenger offload through immigration!
I would suspect that this has changed of late (it has in one respect that EI doesn't serve EWR anymore) but your post seems to imply it hasn't!