It measures roughly 7 metres by 9 metres and rises just over 0.4 metres above the current ground surface, though the full depth of the structure extends considerably further down into the bog. This kind of feature is known as a burnt mound, or fulacht fiadh in Irish, and represents one of the most common yet least understood monument types in the Irish landscape. The leading theory holds that they were used for cooking, with water heated by dropping fire-heated stones into a trough, though proposals ranging from brewing to textile processing to communal bathing have all been seriously entertained by archaeologists.

The mound at Ballynahulla came to light during licensed archaeological testing carried out under the reference 15E0390. Excavators cut a trench through the peat, revealing a soil profile that shifted from brown peat in the upper layers to a darker, older peat below, with the deposit running between 0.6 metres deep at the south-western end and 1.1 metres at the north-western corner, precisely where the burnt mound sat. The mound itself was found to be intact, which is relatively significant given how routinely such features are disturbed by drainage works, turf-cutting, or agricultural improvement across the Irish midlands and west. Nothing else of note was recovered from the trench, leaving the mound's date and precise function, as is so often the case with these sites, a matter of inference rather than direct evidence.