That configuration, which may once have rendered the spot effectively an island, seems to have made it attractive to prehistoric people for a very particular kind of activity, the evidence for which only came to light when a plough broke the surface.

What the ploughing exposed was a burnt mound, a class of site found widely across Ireland and Britain and typically associated with Bronze Age use, though the precise function of individual examples is still debated. The general interpretation involves the repeated heating of stones in fire and their use to boil water, after which the cracked, fire-shattered stones were discarded in a heap. At Greenwood, that process left an elongated arc of shattered stone lying in dark, charcoal-rich soil, stretching roughly 20 metres from south-south-west to north, and about 7.4 metres wide, with a handful of smaller discrete patches of burnt stone a few metres further to the north and north-east. What makes the Greenwood site particularly interesting is that it does not stand alone. Three further burnt mounds lie within roughly 180 metres of it, one approximately 140 metres to the north, another around 130 metres to the east-south-east, and a possible third some 180 metres in the same general direction. Whether they represent broadly contemporary activity or accumulated use of a favoured lakeshore setting over a longer period is unclear, but the clustering around the lough suggests the water itself was a constant draw, likely essential to whatever was being done here.