It's not just Dublin, you have to go through security twice in SNN as well, first when you enter the general departure area and then when you go to enter preclearence section you get checked again.
It's been a few years since I have travelled to the US out of Dublin, but I seem to remember it was a 'reduced' screening after pre-clearance, in that bags and shoes were x-rayed but the other parts were not as thorough as a full screening. Has this been upgraded recently?
There are two reasons for the second screening.
- You have/may have had access to your checked bag, so they want to ensure that you haven't swapped anything that is not allowed in your carry-on during this time
- The original screening did not meet TSA standards, e.g. you did not have to remove your shoes for those that don't meet trusted traveller.
Remember travellers are released directly into the sterile area on arrival in the US. Anyone who doesn't do pre-clearance has to be re-screened on arrival. The double screening could be eliminated, but my guess is it is not deemed cost-effective. It would be a complete waste to create a full shopping area that is only useable during a small portion of the day. Some lounge space is fine, but most retailers would not like to have shops sitting completely idle for much of the day. It might be possible to 'upgrade' the main security to meet the TSA guidelines, but that would not solve the problem of access to checked luggage at pre-clearance.