Slag is the glassy, stony waste left over when iron ore is smelted or worked, and its presence in the soil is one of the more reliable indicators that metalworking once took place at or near a given spot. In itself it is not dramatic material, but it carries the logic of a former working landscape within it.

The slag at Ballykina came to official attention through topographical fieldwork carried out in 1985, which flagged the location as a probable metalworking site. The exact nature of the operation remains unclear; the evidence is suggestive rather than conclusive, and no excavation appears to have followed. What adds a layer of interest is the proximity of a horizontal-wheeled water mill. This type of mill, sometimes called a Norse mill, uses a horizontal wheel set directly in a fast-moving stream, driving the millstone above without the need for gearing. They were widespread in early medieval Ireland and remained in use in the west of the country into relatively recent centuries. Whether the mill and the metalworking site were ever connected in function or in time is not something the available evidence settles, but the combination is suggestive of a locality that once supported more than subsistence farming.