This arrangement is the defining feature of a souterrain, an early medieval underground structure, typically cut from earth or rock, that served variously as a place of refuge, food storage, or concealment. What makes the Bigmarsh example quietly interesting is not just its survival but its geometry: the semicircular plan is a deliberate architectural choice, and the three chambers are joined by what are known as creepways, narrow connecting passages designed to slow or stop an intruder.

The structure comprises three earth-cut chambers of broadly similar proportions. The first measures roughly 2.8 metres in length and 2.3 metres in width; the second is slightly longer at 3.1 metres, with a recorded height of 1.2 metres; the third returns to 2.8 metres in length but is somewhat lower, at around 1 metre in height. Each chamber contains a construction shaft, an opening made during the original building process to allow spoil to be removed from above, and later sealed. The original entrance to the souterrain was located at the south-east end of the first chamber, the typical position for such structures, which were often integrated into the banks or interiors of early Irish farmsteads known as ringforts. The site was documented by R. M. Cleary, whose direct communication provided the detailed dimensions recorded here.