You clear immigration and then they ask you if the picture of your bag is yours and then you go airside at SNN where there are toilets, a duty free shop and seats. We were then pretty much called for boarding right away.
Once at JFK, we just walked off the plane and out to baggage reclaim. No customs or immigration. Very easy.
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 ?