The Irish Whale and Dolphin Group (IWDG) says a swimmer suffered a compressed vertebrae, broken rib and damage to her lungs after being rammed by a dolphin in Doolin in County Clare.
The incident, which happened last Sunday as the woman was trying to climb out of the water, was one of several to involve the animal, called Dusty, in recent weeks.
Following a request from Clare County Council the IWDG has put up posters around Doolin slip and harbour warning about the hazards of swimming with Dusty.
In a statement on its website, the IWDG said: "Clearly this level of injury is very serious and a fatal attack could easily happen.”
“IWDG are concerned that more people may be planning to visit Doolin over the long weekend and as always we request people not to enter the water with this bottlenose dolphin, or any dolphin for that matter.”
“Clearly a contingency plan needs to be developed with the relevant and competent authorities in the near future on management of Dusty and swimmers in Doolin."
Dusty has been in North Clare since 2000, where she was first seen around "the jump" near Doolin.
It is IWDG policy to discourage people from swimming with whales and dolphins in Ireland.