The location and orientation of the passage mound was clearly important, and 'capturing the light of the Moon' may have been the main purpose of the construction - as it is orientated to a hill called Knocknarea, which means "Hill of the Moon", which is in-line with the most northerly point the setting Moon reaches on the horizon, ('lunar standstill'), an event that only happens every 18.6 years (suggesting a long construction or  preparation time)