The alert was issued about the Lion’s Mane jellyfish, whose tentacles can cause temporary pain and redness.
In normal circumstances the stings are not known to be fatal, but contact with a large number of tentacles can lead to medical attention being sought.
Fingal County Council (FCC) in north Dublin has issued a warning to its beach users to be wary and issued pictures, which be feature here, of some of the jellyfish which have appeared recently.
“Important notices regarding Lion’s Mane jellyfish,” said a spokesperson.
“These have been spotted at a few of our beaches over the last couple of days.
“Final County Council is urging bathers to be extra vigilant on all our beaches where Lion’s Mane jellyfish are found.”
They add: “Please note that even when they’re dead and washed up on the beach, the venom stays in their tentacles for a few days.
“With so many trailing tentacles you could still get stung, even when you try not to swim near them.
“Also fragments of the Lion’s Mane jellyfish’s tentacles that break off in the water will still sting you, even if they’re no longer attached to the jellyfish.”
A sting from a Lion’s Mane jellyfish can cause nausea, sweating, cramps, headaches and other symptoms and severe stings should seek urgent medical attention.
“Where the lifeguards have noticed a large number of Lion’s Mane jellyfish, they may raise the red flag to advise against swimming. If you are stung on a lifeguarded beach – please approach the lifeguards on duty for assistance,” FCC urge.
First aid for the treatment of jellyfish stings includes
The council also issued guidance on what not to do.
Lion’s Mane jellyfish use its stinging tentacles to capture, pull in, and eat prey such as fish, plankton and other sea life.
On a July day in 2010 over 150 bathers were stung at an American beach, possibly by the same jellyfish.