Chemical analysis of twenty-six amphibolite facies gneisses from western Connemara are treated statistically, and used in conjunction with petrographic evidence to elucidate their origins. It is shown that the gneisses are of igneous ancestry, and have been invaded by material of silica-soda-potash composition. The conditions of temperature and pressure prevailing at different times is related to the evolved mineral assemblages, and the whole sequence fitted into the general pattern of events in Connemara.