Stings Lion's Mane jellyfish can be deadly, causing nausea, sweating, cramps and headaches.
Fingal County Council released a statement warning swimmers and dog owners of the threat and advising anyone who receives a sever sting to seek urgent medical attention.
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Another jellyfish of this kind was reported in by the RNLI in Lough Swilly, who issued a similar caution.
Lion's Mane jellyfish can growing up to 36 metres long and are usually spotted off the East coast during the summer months.
