The numerous Zn-Pb deposits in the Irish midlands, together with the quantity and grade of the ore, make this a world-class base metal province. Despite significant exploration and research, there is still disagreement on the origin of the mineralizing fluids. In this study, we have measured the composition of fluid inclusions using a crush-leach technique. These data constrain the possible sources of the fluids both during and after mineralization. Chloride and Br concentrations indicate that the main ore fluid at Tynagh and Silvermines was seawater that had evaporated until the salinity was between 12 and 18 wt percent. However, Na, K, and Li data show that water-rock interactions have resulted in the depletion of Na and the enrichment of K and Li relative to the concentrations expected for evaporated seawater. The fluids that produced postore dolomitization were also derived from seawater that had evaporated to higher salinities and had precipitated halite. The concentrations of Na, K, and Li are what would be expected for seawater at this degree of evaporation, and show that these fluids did not interact with the same lithologies as the ore fluids. The major change to their fluid composition was exchange of Mg for Ca during dolomitization. We suggest that the ore fluids could only have reached their high salinity by evaporation of seawater on the extensive shallow water shelf regions that existed over much of the Irish midlands.