In Dublin, Ireland I've worn thongs to White Rock Beach, albeit there is an unofficial nude beach on one side, the rest is primarily regular. And for local pools:
- Mespil pool - Where I first wore a thong in full view of everyone around. It's quite relaxed and quiet and all adults
- Club Vitae - Which is part of a hotel and there was a good mix of people and then families started coming in
- Gym Plus(Clarendon Street) - This was previously Energie fitness. It's closed now and even though they had a ridiculous rule saying no "Snazzy swimwear... only wear conventional swimwear... leave the mankini at home," I wore thongs there
- Markievicz pool - This is in the heart of the city and it a fairly busy local pool with a mix of ages and people. Worn thongs twice now when busy **UPDATE 3rd Nov 2023 - I've gone here one and off throughout the last year or so and just today, an overzealous staff member who was in the pool yelled out at someone. Then a few minutes later a woman staff member comes out and checks the sauna and asks to talk to me. I was told that what I'm wearing wasn't allowed as it might be at the beach and that even woman aren't allowed to wear it [thong] showing the bum etc etc. In fairness the lady was very warm and explained nicely but it was still off putting as I just go to swim and use the sauna and nothing else.
Haven't had the chance to try other beaches or pools yet.