Ireland has an infestation of dangerous lion's mane jellyfish, and five people have needed hospital treatment after being stung by them.
Irish Water Safety said the jellyfish are in our coastal waters weeks, if not months, earlier than normal.
The most recent sting victim was Jack Dunne (14), who was swimming at Port Beach in Co Louth when the "huge" jellyfish stuck to his shoulder and chest.
"Jack was with friends and he's six feet tall and was swimming at shoulder height when he got stung," his mother Mellissa said.
"Its tentacles went around his legs and waist.
"His friends rang me and when we got to the beach, he was on his hands and knees and finding it hard to breathe."
Jack's parents immediately took him to the emergency department of Our Lady of Lourdes Hospital in Drogheda.
Mellissa said that on the journey, Jack was vomiting and in pain.
"By the time we got to the hospital he was starting to lose the feeling in one of his legs with the pain. It was a horrible experience for him," she added.
Jack was given strong antihistamines and anti-inflammatory tablets and was much better the following day.
"He found it hard to sleep that night and was itchy where he was stung," his mother said.
She added that the jellyfish was a lion's mane, which can grow up to 25 metres, and they were also found at Clogherhead Beach, Co Louth.
Mellissa said she would not like to see a small child get stung "or any child for that matter".
"The sting went through his body. He was in excruciating pain and he said he will never swim in the sea again" she said.
Irish Water Safety chief executive John Leech said that "this year is the earliest and largest infestation of lion's mane jellyfish in my experience".
They have also been seen in Galway Bay and are "very, very big and cannot be mistaken for anything else".
"They have very long tentacles and there were a lot in our waters last week because of the spring tide," Mr Leech added.
There are fewer this week, he said, but there have been reports of them at Port Beach and he said Jack "was very lucky".
Four people were hospitalised after getting stung in recent weeks.
Mr Leech also warned that "there is the risk, although small, of someone getting anaphylactic shock" from such a sting.
"Every year people are hospitalised after being stung by this jellyfish," he said.
Lion's mane jellyfish are usually seen in Irish waters in September when the water is warmer.
Sometimes they are here in July or August, "but they are here this year in early July and Jack was the fifth person to be taken to the A&E to be checked out," Mr Leech added.
He said the increased number of jellyfish is because "there are not enough predators to eat them".
The creatures' natural predators are turtles and sunfish, but due to plastic pollution of the oceans there are far fewer of them.
"The turtles are being killed off by our plastic pollution and that increases the number of jellyfish," said Mr Leech.
"It is the most serious problem we have to deal with."
Mr Leech advised people to swim only at beaches with lifeguards because they carry out jellyfish patrols and if they see them they can hoist a red warning flag.
More information can be found at iws.ie.