The European flat oyster Ostrea edulis once settled in high densities throughout its natural range but now exists only in small fragmented populations. In the Sea Lough of Strangford, Northern Ireland, recent increases in intertidal oyster numbers at historical sites along the north-east shore were recorded in 2018. A substantial number of conjoined oyster settlements were recorded within this density increase. One intertidal site produced numerous three-dimensional (3D) O. edulis specific matrices containing >16 oysters. In contrast, an extensive search of post and pre-1700s literature uncovered relatively few accounts of species-specific 3D O. edulis matrices and none relating to intertidal populations. The gregarious 3D settlements discovered during this research represent the first documented evidence of the phenomenon in Ireland. These emergent native oyster reef structures offer an insight into the possible intertidal O. edulis formations, which existed pre-1700 and could act as a guide to what may still be obtainable in the future.