It is barely a metre tall, built from random rubble and propped against a natural boulder face, yet its proportions tell a precise industrial story: a rectangular blast furnace, roughly 1.55 metres north to south and 0.9 metres east to west, with an opening cut into the east wall and a bellows aperture, a channel through which forced air was driven to raise the heat of the fire, measuring just 0.4 metres wide and 0.25 metres high, sliced through the base of the rock beside it. A large stone slab still lies on the interior floor, and scattered rocks between the furnace and the waterfall may once have supported a run of wooden lades, the troughs or channels that would have directed water to power the bellows mechanism.

The furnace at Derrycunihy sits within a wider landscape of movement and industry. About ten metres to the west, a trackway 1.7 metres wide, with stones embedded in its surface, runs northward towards the lakeshore, suggesting an organised route for bringing in ore or charcoal and carrying out finished iron. The dating of the site places it in a particularly charged moment in Irish history. According to the historian Rynne, writing in 2001, the furnace most likely dates to the late seventeenth century, or at the very earliest to the period following 1641, when ironworks across Ireland were deliberately destroyed during the rebellion of that year. The early Irish iron industry had been extensive and controversial; English planters had established large furnaces across the country in the late sixteenth and early seventeenth centuries, consuming vast quantities of woodland for charcoal, which contributed to significant deforestation. Whatever its precise origins, the Derrycunihy furnace was operating in the aftermath of that disruption, in a woodland setting that would have provided both fuel and water power in close proximity.