An adequate understanding of the systematics of endemic taxa is necessary for effective conservation policies. The hard water mussel, Margarit(fera durrovensis Phillips, 1928 is endemic to Ireland but its taxonomic status has been the subject of debate. This study compares populations of Margaritifera margaritifera (from Ireland, Scotland, Wales and N. America) with M. durrovensis from the River Nore. Aspects of shell morphology, shell structure, anatomy and genetics are used in the analysis and the results indicate that M. durrovensis is an ecophenotype of M. margaritifera. The geological history of M. margaritifera in Ireland is reviewed along with the ecology, population size and status of M. durrovensis. Proposals are made for the conservation of the ecophenotype as it is an invaluable tool for the study of ecological adaptation and evolution.