The local county council has issued a red warning
There are concerns part of the 2016 Ironman competition could be in jeopardy, due to jellyfish.
Dún Laoghaire-Rathdown County Council have raised the red warning flag in Sandycove, and put up signs warning swimmers.
They say Lions Mane jellyfish are in the water and have also washed up on the beach.
“Bathers are advised not to enter the water due to the presence of these jellyfish whose sting can be dangerous and cause serious symptoms including nausea, vomiting and respiratory distress,” the council says.
It says the situation is being monitored.
Meanwhile hundreds of people are expected to attend the Ironman 70.3 triathlon in the city on August 14th.
It will see participants run 13.1 miles, bike for 56 miles and swim 1.2 miles in open water at Sandycove.
The organisers of the Ironman event say: “Ironman 70.3 Dublin is aware of the jellyfish warning at Sandycove.”
“We are actively monitoring the situation and will take action if this issue persists over the coming days.”
“The safety of the race participants is of utmost importance to us, and we have steps that we would take, should we need to take action on this issue of jellyfish in the swim section of the race course.”