Bathers along the east coast are being warned to stay away from popular beaches this weekend after an influx of the dreaded Lion's Mane Jellyfish.
Red flags have been raised at Dublin's Sandycove and the popular 'Forty Foot' bathing spot amid warnings that the painful stings can cause severe reactions.
Bathers in Cork and Kerry are also being warned about the potentially dangerous jellyfish in the south-west this weekend.
Irish Water Safety CEO John Leach is advising people to avoid swimming off beaches where there are no lifeguards.
He said: "They are to be taken quite seriously and hence why the lifeguards have unfortunately had to raise the red flags.
"They will lower the flags once the number of them reaches a sufficient level so that they can keep the designated bathing area between the flags clear of the jellyfish, but it's a very difficult thing to do.
"The bell on them is about a diameter of one to two metres and there is many hundreds of tentacles on them."