White-clawed crayfish (Austropotamobius pallipes) is a keystone species found in western European freshwater bodies, where it has suffered drastic declines due to pathogens, competition with non-indigenous crayfish species (NICS) and habitat deterioration. In Ireland, populations of (naturalised) A. pallipes have been considered healthy and abundant mainly because no diseases or NICS have been reported in the past decades. The present study investigated a sudden mass mortality event that occurred in the Erne catchment in 2015. Molecular analysis confirmed that the cause of the event was infection by the oomycete Aphanomyces astaci (the causative agent of the crayfish plague). While in 2015 the spread of the pathogen appeared to remain confined to the outbreak’s epicentre and nearby upstream waters, follow up surveys using conventional methods and environmental DNA (eDNA) approaches indicated that by a year later (2016), the pathogen was still present and had spread downstream beyond Lough Gowna. No NICS were detected during the surveys conducted. This crayfish plague outbreak is of grave concern to Irish white-clawed crayfish and associated ecosystems.