These subterranean structures, found throughout Ireland but particularly common in Ulster, were typically built between the 6th and 12th centuries AD. Souterrains consist of stone-lined tunnels and chambers dug into the earth, often with multiple passages, small chambers, and cleverly designed features like creeps; narrow points that would force any intruder to squeeze through on their hands and knees, making them vulnerable to defenders.

The exact purpose of souterrains like the one at Carnaghan remains a subject of archaeological debate. While they were once thought to be primarily defensive hideaways during Viking raids, modern archaeologists lean towards their use as storage facilities for food and valuable goods, taking advantage of the constant cool temperatures underground to preserve perishables. Some may have served dual purposes, functioning as both storage spaces and temporary refuges during times of conflict. The Carnaghan souterrain, like many others across Ireland, would have been associated with a ringfort or early medieval settlement above ground, forming part of a complex domestic landscape that has long since vanished from the surface.