Lions Mane jellyfish have been spotted off the south Dublin coast at Sandycove.
Warning signs have been put up and swimmers are advised to proceed with caution.
These species of jelly fish can deliver a venomous sting which can cause medical complications.
Dun Laoghaire-Rathdown County Council have said that life guards are keeping a vigilant eye on the water and bathers.
As an added precaution the life guard warning flags are on red and will remainin place till further notice.
It warned that the jelly fish can still deliver a sting after it has washed up on the beach and so caution should be taken both in and out of the water.
If stung and you suffer any serious symptoms including nausea, vomiting or respiratory distress you should seek medical attention immediately.