Have just looked the CWLCY pages on the BA website and they have had a makeover.
Previously there was a lot more info on how the stop at SNN worked (including about a reserved area) but it's all been 'enhanced and improved' so all it says now is
On our way to New York we make a brief stop to refuel at Shannon, Ireland. If you travel on flight BA001 you’ll be able to pass through US Customs and Immigration at the same time, meaning we can land in the United States as a domestic flight and you can bypass New York passport control.
If you are travelling on flight BA003 the crew will offer you refreshments and snacks on board during the short stop and provide you with an iPad for your personal use.
And no mention of Gate 24 at LCY but it's comforting to know that you have
once onboard !