This stone is just inside the graveyard here by Loch Rannoch. I found a photo of it here (a fuzzy photo of it apparently whitewashed and with added photoshop blood) on 'El Vicente's Homepage' where the folklore associated with it is retold in lively style (taken from 'Tales of Rannoch' by A D Cunningham). Well as lively as a story about dashing small children's heads out on rocks can be I suppose. An extra long version is in this edition of the Celtic Magazine from 1887.
It's not on the RCAHMS map at all. But could it still be old? Perhaps someone has seen it in person.