The Druids at Craigh Na Dun (Outlander, S1.1, “Sassenach”)
In Druidry, the days of Samhuinn (October 31 to November 2) represent a time where the veil between our world and the World of the Ancestors is lifted, allowing those individuals who are prepared to journey safely to the other side. Druid rites facilitate contact with the spirits of the departed, who serve as guides and sources of inspiration. Some of these rites include the use of fire and circle dancing around stone circles oriented to their points of sunrise and sunset. (x)
In terms of Outlander, this could explain the presence of Ghost Jamie, to see Claire safely on her journey to the other side and through time. Claire’s ability to travel through the stones implies a Druid ancestry, and perhaps the same applies to Geillis Duncan. The abilities seem to be hereditary.