2006:1163 - DERRINSALLAGH (4), Laois

The site was identified during testing along the route of the M7 Portlaoise to Castletown/M8 Portlaoise to Cullahill motorway scheme. Topsoil-stripping uncovered a complex of archaeological features. In total 41 bowl furnaces associated with metalworking, three charcoal-production pits, four spreads of archaeological material, six linear or curvilinear ditches or drains, 24 pits of unknown function, seven random post-holes and two circular structures were excavated.

The two structures were situated less than 3m apart, on a low plateau to the north of the site, where the soil was drier. Both were subcircular in plan, but their method of construction and size were different. The northernmost structure consisted of a subcircular arc of six post- and stake-holes, with a central post-hole, and measured 5.5m north-west/south-east. Two more post-holes at the south-eastern corner marked the outline of a possible entrance. The southern structure comprised three curvilinear slot-trenches forming an arc. It measured 8m in diameter. No hearths were found in association with either structure.

The bowl furnaces occurred singularly and in complexes of two, three, or four grouped closely together, and were dispersed throughout the site. The dimensions differed for each individual furnace, but they were mostly subcircular or ovoid in outline. The smaller bowl furnaces measured on average 0.2–0.3m in diameter and were 0.1–0.25m deep; the larger furnaces were 0.4–0.9m by 0.25–0.4m deep. Each of the bowl furnaces had an outer ring of bright red oxidised clay surrounding varying amounts of vitrified material. The deposits within each furnace differed only slightly, the majority of deposits consisting of charcoal-rich black silty clay that usually contained varying quantities of iron slag. The larger bowl furnaces had substantial amounts of vitrified material lining the edges of the cut. The bog ore used in the bowl furnaces was likely to have been sourced locally. The charcoal produced from the three production pits excavated on site was probably utilised in these bowl furnaces. The non-industrial pits may have been used for domestic refuse. The linear and curvilinear ditches appear to have been contemporary with the activity on site. The largest of the ditches was situated to the south of the site but extended beyond the limits of the excavation. It was excavated for a length of 35m in an east–west direction and was 1.3 wide by 0.65m deep. A large quantity of slag was recovered from the fill of the ditch.

Provisional radiocarbon dates obtained from samples from two bowl furnaces places the ironworking activity at Derrinsallagh 4 well within the Iron Age period. Post-excavation analysis is still ongoing for the site.

Unit 21, Boyne Business Park, Greenhills, Drogheda, Co. Louth