It is a souterrain, a type of structure built during the early medieval period in Ireland, typically by hand-digging a trench, lining it with drystone walls, roofing it with large capstones, and covering the whole thing over with earth. These passages were associated with nearby settlements and ringforts, and their purpose, still debated, likely combined storage of perishables in the cool underground air with a degree of refuge or concealment in times of trouble.

The Tonrevagh souterrain is recorded as a monument, placing it within a long tradition of such structures found across Connacht and beyond. Galway's landscape contains numerous examples, many of them discovered by chance during agricultural work or drainage schemes, and a good number remain only partially investigated. Without fuller documentation in the public domain, the specifics of this particular structure, its dimensions, its condition, its relationship to any surrounding settlement features, remain difficult to establish with confidence.