Mussels belonging to the Genus Mytilus are a significant ecological and commercial species of nearshore marine habitats. Along the south, west and north coasts of Ireland, intertidal mussel populations consist of the native blue mussel Mytilus edulis, non-native Mediterranean mussel Mytilus galloprovincialis and hybrids of both while M. edulis is exclusive to the east coast. M. galloprovincialis is listed in the ‘‘World’s worst 100 invasive species’’ as it has outcompeted native mussel and bivalve species worldwide. The objectives of this study were to sample 16 mussel populations (n=471), historically investigated over two decades as well as new populations, and determine species and hybrid distribution and abundance (%) using PCR. The length and weight frequency distributions were also investigated to provide information on size patterns across different coastal locations and wave exposure levels. Mussels were absent from two sites where they were present in a past study. Overall, M. galloprovincialis was dominant at most sites, similar to two decades earlier. At a marine nature reserve, hybrids remained dominant. Mussel species/hybrid diversity and abundance remained stable at two Irish sites, while a “Boom-Bust” dynamic existed at eight sites between M. edulis and M. galloprovincialis. This instability of species composition and abundance may be due to natural phenomena, such as the Atlantic Multidecadal Oscillation (AMO), or natural fluctuations and adaptation to localized environmental parameters. What is evident is that although M. galloprovincialis may “disappear” during particular years and for prolonged periods of time, hybrids still remain and the non-native species reemerge to dominate.