This is a souterrain, one of thousands of dry-stone underground galleries built throughout early medieval Ireland, typically beneath or beside a ringfort, the circular enclosed farmstead that was the standard unit of rural settlement from roughly the sixth to the twelfth century. Souterrains served various purposes, most probably as cool storage for dairy produce or as refuges in times of threat, and their association with a ringfort's hut sites is entirely characteristic.

What makes the Garrane example quietly interesting is the small documentary trail attached to it. The local antiquarian James Grove White, whose multi-volume survey of County Cork was produced between 1905 and 1925, visited and noted an entrance to what he called a cave, partially blocked with loose stones but, in his words, very easy to clear. That casual observation, recorded in the third volume of his work, is now the most detailed eyewitness account available. By the time more systematic survey work took place, the picture had changed: a depression and an opening visible outside the northwest wall of the western hut site suggest that at least one underground chamber has since given way, the ground above it settling and sinking as the corbelled or lintelled roof below gradually failed. The entrance Grove White thought so approachable may now lead only into rubble.