Genetic models for many Irish Lower Carboniferous Zn + Pb ore deposits invoke a dual supply of sulphide. The dominant source is from the bacteriogenic reduction of Lower Carboniferous seawater sulphate, but a significant, minor supply is derived from deep-seated sources. The δ34S range of the latter component varies among the deposits: from the lightest range of —15% to 0% at Keel, to the heaviest of –4% to +14.4% in the Navan/Tatestown area. We hypothesize that such sulphur is leached mainly from diagenetic sulphide minerals in the underlying Lower Palaeozoic sediments. 6”s of pyrite in the Silurian/Ordovician Moffat Shales (—17.1% to —0.6%), and of sphalerite and galena in Lower Palaeozoic-hosted veins at Salterstown (—8.7% to -4.5%) and Wanlockhead (—10.3% to —5.1%) are consistent with the hypothesis. Below the Navan ore deposit, Lower Palaeozoic shales containing minor diagenetic pyrite with a very wide range in δ34S have been found; δ34S is typically heavy (+16%) but extreme values up to +62% are encountered. The general enrichment in δ34S is in accord with the noticeably heavy isotopic composition of deep-seated sulphur in the Navan orebody and its Tatestown satellite. Our preliminary results therefore suggest that geographical variations in the δ34S range of Lower Palaeozoic diagenetic -write_. may have contributed to the isotopic variation in the deep-seated sulphur among the Irish deposits.