Sometimes, if I park somewhere I think is a bit 'iffy' - a farmer's field or something, I put a note in the windscreen saying 'gone to look at stone circle' and include my mobile number. If there's no-one to ask, and no-one about, I think it's only courtesy if there's a possibility you're on private land.

Obviously, I never block roads or entrances.

I've been all over Ireland and Scotland looking for circles, and have never EVER met with any farmers/land owners that were hostile to me. Most are happy to chat and often, I suspect, quite proud they have something on their land worth visiting.