Our knowledge of non-native algae in benthic estuarine habitats is relatively poor, especially compared to algal introductions along open shores or on floating structures. Gracilaria vermiculophylla is a widespread macroalgal invader in the temperate estuaries of the Northern Hemisphere, and, here, we expand its documented range within northeastern Ireland and England. Established populations occur within two inlets in the border counties, Carlingford Lough (Counties Louth and Down) and Dundrum Bay (County Down), but G. vermiculophylla is absent from open coasts between these sites. Repeated surveys in Dundrum Bay showed variable abundances, with an increase in biomass between 2013 and 2016. Three populations were discovered in England, where this species had not previously been identified: Christchurch Harbour (Dorset), Brownsea Island in Poole Harbour (Dorset), and Kingsbridge Estuary (Devon). The Irish and English thalli belong to the most common, invasive cox1 haplotype 6. Using a combination of morphological observations and 10 microsatellite loci, we found that the population at Carlingford Lough included both reproductive haploid gametophytes and diploid tetrasporophytes and genetic signatures of sexual reproduction, but the populations at Christchurch and Brownsea displayed signatures of partial clonality. Genetic diversity was higher along the south coast of England as compared to the Irish population, consistent with patterns of diversity previously described for the European coasts. Finally, we also note the occurrence of a putative G. vermiculophylla population in Wales at Porthmadog, Gwynedd. As the sites in which we have now documented G. vermiculophylla in the British Isles also host shellfish aquaculture activities, our study is further evidence for the role of aquaculture in the spread of invasive species.