In Dublin, once you go through security, you are directed (via Duty Free and shopping, of course) to the Pre-clearance area which is near the boarding gates and, if my memory serves me correctly, is on a lower floor than the shopping area. Once cleared you cannot go back as technically you are now on American soil. This means that the flight out of Dublin becomes a domestic US flight (even on EI) and duty free is not available for purchase onboard. Whether there is duty-free, or indeed any shops other than ones similar to WHSmith, etc. I do not know and my guru is currently in the US so you will have to await his return to find out. He's back next week

I have also been pre-cleared leaving Canada from both Calgary and Vancouver. From Calgary, it was very smooth with just going through security and the Immigration checks as per normal. There really was nothing to differentiate it from an internal US flight apart from that. In Vancouver it seemed far more aggressive. For starters, there was a dedicated check-in area for all US flights which took up a whole wing of the International terminal. Once past check-in there were large signs announcing that you were now entering the United States of America and it was the full-on experience we have come to expect when going through security in US airports. Once past that and Immigration there was a small duty-free area and some shopping but as my gate was an afterthought for the turbo-props that just hop across the border to SEA and other similar airports it was hard to judge.