Illite crystallinity studies on metaclastics from the Variscides of SW Ireland range in value from 0.18 to 0.25 degrees Delta 2 theta indicating a metamorphic grade in the upper anchizone-lower epizone (c.275-325 degrees C). Chlorite geothermometry yields a similar metamorphic temperature range (280-315 degrees C). Combining these data with known overburden estimates for the area (c. 5 km) implies late Palaeozoic geothermal gradients in excess of 60 degrees C km(-1). Fluid inclusion studies on quartz veins in these metaclastics reveal two vein types characterized by fluid densities of 0.7-0.93 g cm(-3) (Group 1) and 0.9-1.0 g cm(-3) (Group 2) respectively. This variation in fluid density is thought to be dominantly controlled by temperature, with Group 1 veins linked to late Palaeozoic extension and high geothermal gradients and Group 2 Veins associated with subsequent Variscan deformation and lower temperature conditions.