Dun Laoghaire-Rathdown County Council has put up warning signs in the lifeguarded area of Sandycove in south Dublin after sightings of lion’s mane jellyfish in the water.
Bathing has not been prohibited as it was with the water quality warning earlier this month.
As an added precaution the lifeguard warning flags are on red and will remain in place till further notice, the council says.
Lion’s mane jellyfish carry a painful and potentially lethal sting, even when washed up on the beach.
Last summer saw more than 150 lion's mane jellyfish recorded along the West Coast — amid a spate of incidents with swimmers being stung in Galway Bay.
But the East Coast was not immune, as a teenager was hospitalised after he was stung over most of his body off a Co Louth beach.