It is a souterrain, a type of artificial tunnel or chamber built during the early medieval period in Ireland, typically constructed from dry-stone walling and covered with large capstones. These structures served various purposes depending on the site, including storage, refuge, or as annexes to nearby settlement enclosures, and they turn up across the Irish landscape with a frequency that still surprises. What makes the one at Shanvallyhugh notable is less any dramatic feature than the quiet fact of its existence in a part of Mayo that preserves a good deal of early medieval activity beneath its surface.

Beyond its classification and location, documented detail about this particular souterrain is currently sparse. The place name Shanvallyhugh is itself worth a moment's attention. Names beginning with "Shan" often derive from the Irish "Sean," meaning old, while "bally" typically points to a townland or settlement, suggesting the area carries a long memory of habitation even if its specifics have not been fully worked through in accessible records. Souterrains of this kind are generally associated with ringfort settlements, the circular earthwork enclosures that were the dominant farmstead form in early medieval Ireland, roughly between the sixth and twelfth centuries. Whether a ringfort survives in the immediate vicinity at Shanvallyhugh is not confirmed in what is currently available.