Structures of this kind are known as souterrains, a term borrowed from French meaning simply "underground passage", and they appear at hundreds of early medieval Irish sites, most often associated with ringforts, the circular enclosed farmsteads that dot the Irish countryside. Their precise purposes are still debated, but storage, refuge, and the cool preservation of dairy produce are the explanations most commonly offered.

The Mocollagan example is modest in scale but specific in its details. The first chamber, entered from the ringfort interior, measures roughly 1.6 metres long, 1.7 metres wide, and just 1.1 metres high. A creepway at its northern end is now blocked. The second chamber is considerably longer at 4.9 metres, roofed by nine capstones, and terminates at the outer face of the inner bank, where access through a hole in the back wall brings a visitor back into the open air. The whole arrangement, with its carefully laid stonework and its deliberate routing beneath the bank itself, suggests a structure that was built not as an afterthought but as an integral part of the ringfort's design. The site was recorded by D. Lavelle in a 1994 archaeological survey of the Ballinrobe district, covering the area around Lough Mask and Lough Carra.