I visited this site as I was at Tara on Saturday:
http://www.themodernantiquarian.com/site/2711
There were 'offerings' here, some of the more modern religious variety and some of the new age variety. They in no way detracted from the beauty of the place. In fact, in a way I'm glad they were there. I'm glad people still bother with this, interact and be creative with their (dare I say it) sacred places.
However, this is a well-tended site. The locals obviously love and look after it. Should hordes of the great unwashed show up and do their thang, I don't have to tell you what the response would be.
What can we do? Clean up and keep cleaning up and do some more cleaning up. When we're finished we could always clean up some more. :-)