Buried beneath what is now the M4 motorway corridor in County Kildare, a small early medieval enclosure once held a working iron forge. The site at Cappagh came to light in 2002 during archaeological excavations carried out ahead of the Kinnegad-Enfield-Kilcock Motorway Scheme, and what it revealed was a self-contained industrial operation from Ireland's early medieval period, compact enough to fit within a single enclosure yet sophisticated enough to suggest specialist knowledge at work.

Within the enclosure, excavators identified three structures alongside a dedicated metalworking area. Several bowl furnaces were found, shallow depressions used for smelting or smithing, along with curvilinear gullies still carrying metallurgical waste. At the eastern edge of the site, a possible shaft furnace was tentatively identified; this type of furnace, taller and more enclosed than a bowl furnace, allows for greater heat generation through a controlled air supply. The feature was figure-of-eight in shape and contained dense concentrations of slag, the glassy residue left over from smelting, as well as furnace bottom, the solidified material that collects at the base of a furnace after repeated firings. Charcoal packed into one end of the feature would have served as the primary fuel. The raw material feeding all of this activity may have come from a nearby bog. Bog iron forms naturally in wetland environments when iron minerals precipitate out of groundwater, and it was a commonly exploited resource in early medieval Ireland precisely because it required no mining. Fragments of what appears to be unprocessed bog iron were recovered from several features on-site, a detail that awaited laboratory confirmation at the time of excavation. Supporting the idea that finished objects were made here rather than elsewhere, excavators also recovered a possible mould or crucible, furnace bases, and traces of two or three tuyères, the ceramic or clay nozzles through which air was blown into a furnace to raise the temperature. Together, these finds point to a site where iron was not merely worked but likely produced from scratch, from raw bog ore to finished object, within the same small enclosure.