Can reality intrude here for a moment? Shannon isn't going to become an international passenger hub. If my flight had to land somewhere to clear US customs before entry in the US I would choose another flight as would the vast majority of travellers. It's a no brainer.
The *ONLY* reason BA are using SNN is because of the performance limitations from a short runway in London. The region does not have enough population, connectivity, industry or wealth to maintain an international hub operation. It struggles to make a low cost point to point operation work in Ryanair and that in itself is an achievement!
Transatlantic transit traffic is an evil neccesity from another age and is almost dead in modern international airlines. The only people who need it are the military and those niche carriers flying aircraft beyond their usual routings or flying aircraft that ought to be parked in the desert by now.
I mean for God's sake, you can't have a European hub that far from the centre of Europe !