These are among the more enigmatic monuments of prehistoric Ireland, broad low mounds of heat-shattered stones that accumulated when people repeatedly heated rocks in fire and plunged them into water-filled troughs or pits to bring the water to the boil. What exactly that boiling was for, whether cooking, bathing, industrial processing, or something else entirely, is still a matter of debate. What is not in doubt is that these features were used repeatedly and over long periods, and that the two at Ballynattin were no exception.

Archaeologist Anne Connolly excavated both mounds as part of the bypass salvage work. The smaller of the two was a subcircular spread roughly seven metres across and only about twenty centimetres deep, from which a single fragmented piece of worked flint was recovered, a modest but telling trace of human hands. The second mound was considerably larger, stretching more than twelve metres at its widest and reaching a maximum depth of nearly half a metre. Associated with it was a small wooden platform made from narrow lengths of timber, covering an area roughly two metres by one and a half metres, beneath which a series of wooden stakes had been driven into the ground without forming any obvious pattern. A timber sample from the platform was analysed by dendrochronology, a technique that dates wood by matching its growth rings against established sequences, and the result was precise enough to place the felling of the tree in late 856 BC or early 855 BC. That kind of resolution is rare in prehistoric archaeology, and it fixes this modest wooden structure to a specific winter on the cusp of the ninth and eighth centuries BC, when people were working and cooking and heating water in this corner of what is now County Wicklow.