9Van Hoek, M.A.M. "The Prehistoric Rock Art of Co. Donegal (Part II)." Ulster Journal of Archaeology Third 51 (1988): 25.
The diagram of suggested Beltany alignments is from Boyle Somerville, "Instances of orientation in prehistoric monuments of the British Isles." Archaeologia 73 (1923): 193–224, Fig. 11.