I disagree that Sharon was ever a classic name, or Michelle either. In the US, they seemed to come out of nowhere and were very fashionable for a short few decades, then disappeared (Sharon moreso than Michelle). I think the same sudden arrival and almost complete disappearance happened in Britain (and Ireland). And maybe it was Grace Kelly's Tracey who popularised Tracey?
I don't know how Sharon first appeared, but I have a suspicion that the Beatles' 'Michelle' gave Michelle a boost.
I had a friend in the US who went to see The Snapper with her DH while pregnant and came out with the name Sheridan number one on their list of girls' names. Later, after they had their little Sheridan they eventually got around to reading the book and realised that the name they had misheard was Sharon ('Shardn' in Dublinese)
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Yummy -- I'm also Irish, and went to school with loads of Lindas, Lauras, Geraldines, Michelles, Sharons, Karens, Emmas, but also loads of Niamhs, Orlas, Grainnes, Eimears, Aislings, Fionas, Muireanns, Caitrionas; little Sharons are scarce on the ground but there are loads of Niamhs, etc., still (maybe not so many Grainnes?). I think the Irish names have become classics in Ireland. However, I think Siobhan and Sinead suffered a different fate, mainly because when they spread to the UK they landed in the same parts of Britain where Sharon and Tracey regined supreme.