The mass of jellyfish covered an area up to 10 square miles up to 35 feet deep, The Times Online reported Wednesday.
The natural disaster, which occurred at Glenarm and Red bays near Cushendun, jeopardizes the survival of the Northern Salmon Co.
"We are still assessing the full extent, but it's a disaster," John Russell, the company's managing director, told the Times Online.
"In 30 years, I've never seen anything like it. It was unprecedented -- absolutely amazing. The sea was red with these jellyfish and there was nothing we could do."
Scientists attribute an increase in the mauve stinger jellyfish in northern waters to the warming of the seas and a decline in their predators, including sunfish and trigger fish, through over-fishing.