The warm summer waters around Ireland's coastline bring with them a host of summer visitors called jellyfish. Jellyfish are primitive animals with limited powers of movement and so are often washed up on the beach. Shaped like a simple sack, their main means of capturing prey is through the primitive stinging cells on their tentacles.
The stinging cells are triggered by an object touching a microscopic hair. This hair triggers the cell to squeeze violently, ejecting a hollow thread that penetrates the victim's skin and injects poison. In the majority of jellyfish, the effect is so mild as to pass unnoticed. But with some of the larger jellyfish, such as the Compass Jellyfish and, particularly, the Lion's Mane Jellyfish, the sting can cause severe pain and might trigger a dangerour allergic reaction leading to anaphyactic shock, which will require immediate medical attention.
If you are stung by a jellyfish, rinse the affected area with saltwater to dislodge the stinging cells and then neutralise their poison with vinegar. In severe cases, however, seek immediate medical attention.
The best way to avoid being stung by a jellyfish it to keep well clear of them, whether or not they appear alive or dead. In the water, the thin, semi-transparent tentacles my extend much further than you think.
On the beach, even if the jellyfish looks dead, the primitive stinging cells may still be alive and working. So leave them well alone!
The Lion's Mane jellyfish is notorious enough to have featured in a Sherlock Holmes story by Sir Arthur Conan Doyle where Holmes, holidaying in Cornwall, is asked to investigate the apparent murder of a swimmer who looks as if he had been whipped to death.
"The culprit," says Holmes. "Is a jellyfish!"