Three UK have infrastructure in place to DPI traffic to detect tethering and if it's not enabled on your Three UK account plan then the tethered device doesn't get internet access.
Three Ireland currently don't do this for Three Ireland accounts, though tethering is prohibited on All You Can Eat by policy, not by network infra.
As usual, the Three page is less than helpful... I don't know if it's my plan (Classic Flex - Max), I can't get to the terms and conditions and I can't check if tethering is enabled for my account
All I can say is that I've been to the UK and used my laptop with no issues and that I'll try in Spain in a couple of months.
Anyway, you are much more informed than me in this area, so I'll shut up now :)
Can confirm. I was in Hong Kong a few weeks ago and got a "Sorry tethering isn't allowed" message when trying. I think I had to restart my device to get access again as everything was blocked.
In the end I just got a local Three SIM, I think it was ~£6 for 700mb. Trying to find free wifi (without 30 minute access restrictions) in Hong Kong was impossible...
Three Ireland currently don't do this for Three Ireland accounts, though tethering is prohibited on All You Can Eat by policy, not by network infra.