Bottlenose dolphin group size is known to be determined by food availability, social interactions and predator defence. This paper analyses data gathered over seven years from dolphin tour boats operating in the Shannon Estuary and examines whether there were any temporal or spatial trends in group size of the resident group of bottlenose dolphins in the region. Findings indicate that dolphin group size varied significantly between years (ANOVA, F 4.55, P 0.0001), and increased during the months of July, September and October (ANOVA, F 8.921, PB0.0001). Findings also reveal that group size of dolphins encountered in the middle part of the estuary was greater than the outer or inner estuary (ANOVA, F 4.176, P B 0.001). The seasonal change in group size is thought to be primarily caused by dolphins switching to different prey species.