Coastal ecosystems are particularly vulnerable to alien invasions. Regular, standardized, targeted monitoring of coastal areas helps to detect the arrival of non-native species early, identify sites most vulnerable to invasion, and assess potential for further spread. This study quantified the spread and changes in distribution of non-native oyster, Crassostrea gigas, populations around the coast of Ireland. In total 37 sites were surveyed, in areas which either currently or previously harboured cultivated C. gigas, for the presence and abundance of ‘wild’ C. gigas. Wild populations were identified at 20 sites and at four additional sites C. gigas was observed as recently discarded from aquaculture activity. Five of the invaded sites were identified as being highly suitable for a population expansion based on their current population status. Importantly, we also identified individuals of C. gigas and native European oysters, Ostrea edulis, co-occurring within the same shore at five sites. This is the first record to our knowledge of such co-occurrence within Europe. This evidence of co-existing oyster species raises concerns regarding the potential impact of C. gigas on recovering O. edulis populations. In Ireland, however, C. gigas does not typically spread extensively from introduction points, and although self-containing populations exist, they are currently sustained at a much lower density than those observed in other regions such as the Wadden Sea or French Atlantic coasts. We suggest, therefore, that to protect native oyster populations, C. gigas should be eradicated where co-occurring with O. edulis and recommend continuous monitoring of invaded sites.