Interesting. I visited Dingle in May and it dropped off my list of ever recommending to anyone. It was 100 percent tourist trap. The only sights that merited a visit were the two Oratories. I had planned to stay for two nights but left after one, I was so disappointed. I think staying on the Kerry Peninsula offers better sights and few tourists in comparison (because most people drive the loop but don't stay overnight).
There are a lot of must sees in big cities. They're listed everywhere. I think people need suggestions for great things that most people don't see. Unfortunately, most of my travel so far has been the big city must sees. Here's some others I'd suggest:
Spain: Fire festival in Valencia (March), Holy Week (not in Seville, go to Valladolid or Zaragosa), April Faire in Seville.
Germany: Go to a beer fest other than Oktoberfest. I'd suggest Pützchens Markt in Bonn or similar. I also thought Wuerzburg had a lot to offer for a relatively small city.
France: A pardon in a smaller town in Brittany. I love Cairn Gavrinis and enjoyed Vannes, Auray, Locronan and Dinan. I'd suggest Ft. La Latte as a good castle visit. Anywhere in Languedoc area, Foix and Najac (both nice castles) were two of my favorites. I liked Cordes sur Ciel in June, but it may be swamped with tourists in the summer. In the Lot Valley, Pech Merle allows you to still see real cave art. I consider Conques to be head and shoulders above Rocamadour. I also enjoyed St. Cirque Lapopie. In Dordogne, I liked Domme and Sarlat the best (but those aren't necessarily off the beaten path). St. Emilion exceeded my expectations (again maybe not in mid summer?). In Burgundy, I'd recommend Semur en Auxois but it may be best to find your own little town that no one recommends to tourists. In Alsace, the Chateau du Haut Koenigsbourg is a good visit.
In Italy, steer clear of Lucca and (in summer) Cinque Terra. Neither is nice enough to justify the crowds they get. Instead try Siena and Orvieto. Nowhere I've visited (so far) in Italy seems immune to tourist crowds.
In Poland, I'd suggest Malbork (Marienburg) Castle near Gdansk.
In Ireland, the Antrim coast was what I expected and worth visiting. After that I'd focus on the Northwest (Sligo and Donegal areas) but only choose random towns that aren't in guidebooks so you can really experience Ireland.
Scandinavia. The nice thing about Scandinavia is it really isn't mobbed with tourists. I liked Copenhagen as the city with the most to see and do. Stockholm is probably the prettiest (be sure to arrive and/or leave by boat). Again, I still have a lot of exploring left to do.
I haven't been to Croatia, Slovenia, Montenegro or much in the eastern side of Central Europe. After Ireland, my current thinking is to avoid any of the "suggested" big name places and just explore smaller cities/towns to see what I find.