The clue is in the material: lumps of vitreous slag, the glassy residue left over from smelting iron, sitting at the base of a steep four-and-a-half-metre riverbank and, at certain points, on the river-bed itself. It is an unlikely industrial relic for this corner of Kerry.

The ironworks at Gortamullin was one of the more unlikely ventures of Sir William Petty, the polymath surveyor, physician, and economist who acquired a considerable tract of land along the northern shore of Kenmare Bay after the Cromwellian confiscations of the 1650s redistributed Irish land on a vast scale. Petty channelled some of that windfall into the Glanaroughty ironworks project, of which the Gortamullin furnace formed a part. By 1672 the furnace was productive enough to turn out eleven or twelve tons of bar iron each week, a respectable output for the period. Sometime after 1677, however, the works were shut down. They never reopened. The reasons are not recorded here, but the enterprise fits a wider pattern of ambitious post-Cromwellian industrial schemes that foundered against the realities of remoteness, fuel supply, and uncertain markets. What remained, once the furnace itself disappeared under later construction, was the slag: the industrial waste that smelting operations always leave behind, durable enough to outlast everything else.