These are burnt mounds, known in Irish archaeology as fulachtaí fia, and they are among the most common yet least understood monument types in the Irish landscape. The basic principle involves heating stones in a fire, dropping them into a water-filled trough to bring it rapidly to the boil, and then discarding the cracked, spent stones nearby. Over generations of repeated use, these discarded stones accumulate into the low, often horseshoe-shaped mounds that survive today.

Excavation at Ballynabarny, carried out under licence in 2002, uncovered not one but three distinct burnt spreads across the site, suggesting repeated or prolonged activity in this area. The first, a semi-circular mound measuring roughly 8.7 metres east to west and 5.6 metres north to south, sat above a series of depressions thought to be sockets where upright stones once stood. A partially timber-lined trough, about 2.2 metres long and just under half a metre deep, lay close by to the southwest. A second, larger mound upslope measured over ten metres in length and was accompanied by two troughs, the bigger of them partially ringed by large granite boulders, perhaps to stabilise the structure or channel water. A third, narrower mound of similar length completed the spread. The granite boulders are a local touch; Wicklow sits on one of Ireland's most extensive granite formations, and the material would have been readily available for construction and for heating alike, though repeated thermal shock eventually renders any stone useless and fit only for the midden.