I think the requirement to route transatlantic through Shannon only came about when the current main runway opened in 1989. Before that the older runways did not have the length to permit such flights, so it was not an issue, as Shannon stops were required anyway. The duty free emphasis was much older, and developed in the early 1950 days when many carriers stopped over for an hour to refuel both ways, and passengers could make a quick visit to the terminal.
There's a wonderful and surprisingly extensive museum of earlier transatlantic flight days at Foynes, on the opposite side of the estuary to Shannon airport, where the old WW2 flying boat terminal was. It concentrates on the 'boats', but also on the Shannon duty free history, and the barman there inventing Irish Coffee, to warm those arrived from poorly heated piston-engined aircraft. I don't know how much of it is "never let the facts get in the way of a good story", but it's a visit for aviation aficionadoes I very much recommend.