It is a souterrain, a type of structure built during the early medieval period in Ireland, typically consisting of one or more dry-stone chambers and connecting passages dug into the earth or cut into rock. Scholars have debated their precise function for generations; current thinking suggests they served a combination of purposes, from cool storage for dairy produce to places of refuge during times of threat. That such a structure exists at Pollanaskan is, in itself, a quiet reminder of how densely layered the Irish landscape remains.

Beyond its classification and location, the particulars of this souterrain remain difficult to pin down. The source material available at present offers no excavation records, no description of the chamber dimensions, no record of finds, and no detail of when or by whom it was first noted. What can be said is that souterrains of this kind are found throughout Mayo and the broader west of Ireland, often associated with ringfort settlements, those circular enclosures of raised earthen banks that once formed the basic unit of early medieval rural life. Whether this one sits within or near such a settlement is not currently documented in accessible records. The townland name itself, Pollanaskan, carries the Irish roots suggesting a hollow or pool, the kind of small topographical detail that occasionally points toward the landscape conditions that made underground construction practical in the first place.