Identification of the relevant forcing factors behind coastal change is a major societal demand in the face of rising sea level. Changes in the patterns of coastal erosion and accretion at a century scale in Dundrum Bay, Northern Ireland, are here attributed to natural changes in the nearshore bathymetry of sufficient magnitude to alter incident wave-energy dispersal. Simulations of wave propagation across the nearshore reveal a marked change in the nearshore patterns of energy dispersal and related sediment- transport pathways between 1861 and 1968. The 1861 bathymetry is associated with a simulated longshore drift divide in the middle of the bay and with sediment dispersal to the southeast and northeast, whereas the 1968 bathymetry is associated with a consistent northeastward drift throughout the bay. These changes are consistent with recorded shoreline changes: sediment accumulation and foredune accretion in the northeast and northeastward spit elongation across a tidal inlet. This study reveals a case in which purely natural changes in the seafloor at a century scale can be related to long-term shoreline changes. Similar changes during relative sea-level rise during the Holocene were potentially capable of significant modification of shoreline morphology and dynamics. Nearshore extraction of sand for beach nourishment purposes may have consequences of similar magnitude for adjacent beaches.