I have no issue doing the Shannon Shuffle. It is quick, efficient and painless. And, the US Immigration officers in SNN seem to be amongst the nicest and most welcoming around!
The terminal you saw is the new and refreshed one. I flew some of the early services before and then during renovation, and it was a huge nod to retro-70s! Apart from UA / DL services to the US, quite a few US troop transits pass through SNN (usually via Omni Air International).
The best part of the BA experience is the continuing flight - everyone is so relaxed on board, especially upon landing at JFK - no scrum rush to immigration, just a quick stroll to pick up bags. Simples!