The site at Tonybaun is not marked by any obvious monument above ground, but beneath the soil, archaeologists found the remains of a working Iron Age furnace complex, one of the better-documented examples of early metalworking to have emerged from the western counties.

The site came to light not through deliberate search but through the practical business of road building. Archaeological testing carried out in advance of construction work on the N26 revealed a cluster of features that pointed unmistakably to iron production. Three furnace pits were excavated, each containing charcoal-rich soil mixed with iron slag, the glassy waste material left over when iron ore is smelted at high temperature. One of the pits was particularly well preserved, with stone-lined sides and a flat slab set as a base, a construction detail that suggests a degree of deliberate engineering rather than improvised use. Radiocarbon dating placed this pit within the period 477 to 210 BC, while another pit produced dates ranging from 166 BC to 25 AD, meaning the site was active across several centuries, straddling the middle and later Iron Age. Associated spreads of burnt material and slag around the pits suggest repeated use rather than a single episode of activity.

About forty metres to the south of the metalworking area lies a burial ground, one that was later used as a children's burial ground, a type of site known in Ireland as a cillín, where unbaptised infants were traditionally interred outside consecrated ground. Whether the proximity of the two sites carries any significance is not clear, but it gives the low-lying riverbank at Tonybaun an unusual density of early human activity, industry and burial quietly layered into the same unassuming piece of ground.