It has long been held that mythical peoples, such as the Formorians & Tuatha da Dannan in Ireland, are memories of ancient races.

The problem with the Irish myths is that they tell of five invasions, while modern theory doesn't support large scale invasions into the British Isles.

As for thrown stones, I'm not sure. I just think they're ancient boundaries. Then the tale comes along that So-And-So placed the stone there to keep Such-And-Such out. This in turn mutates to become So-And-So threw this stone at Such-And-Such.