This appears to describe no customs check at all, which seems unlikely. Where do they pick up that you bought 20 bottles of liquor in the Shannon duty free, or in the aircraft ?
Sales on board are restricted for pre cleared flights, cigarettes and alcohol are just some duty free items which cannot be sold. Sales after you clear in SNN are restricted per passenger.
Something a loco's will have to balance between additional costs and loss of on board revenue before using such a facility.
Passengers are disembarked via an Airbridge in Shannon, bags are not removed but a photo of your bag will be taken in LCY and you will asked to confirm that that is infact your bag while going through preclearance.
Bags are removed, scanned and photographed and if required an officer can have you and the bag re-checked if necessary before re boarding.