It is a souterrain, a type of artificial tunnel or chamber typically built during the early medieval period in Ireland, roughly between the sixth and twelfth centuries. Constructed from dry-stone walling and covered with large capstones, souterrains were dug adjacent to, or beneath, settlement sites and ringforts. Scholars still debate their precise purpose, though storage of perishables, refuge during raids, and simple concealment of valuables are the most commonly advanced explanations. The Kerry landscape contains a notable number of these structures, which is perhaps unsurprising given the density of early medieval activity in the region, but individual examples like this one rarely attract much attention beyond the record that marks their existence.

The specific history of the Ballygarran souterrain, including when it was first recorded, its dimensions, its condition, and whatever associated features may survive nearby, remains thinly documented in publicly available sources at present. What can be said is that the townland name itself, Ballygarran, likely derives from the Irish baile an gharráin, meaning the settlement of the thicket or shrubbery, which hints at a landscape that would have been at least partially wooded in earlier centuries. Souterrains of this period were almost always associated with a ringfort or ráth, the circular earthen or stone enclosures that served as farmsteads for early medieval families, and it is reasonable to expect that some trace of such a settlement exists or once existed in the vicinity, even if ploughing or other land use has reduced it over the centuries.