Hundreds of young Common Terns were eaten alive by the rats in recent weeks in an attack that has shocked conservationists.
The rats would have had to swim more than 300m to reach the birds before scaling 10m-high wooden supports.
Birdwatch Ireland spokesman Niall Hatch said the scene on the man-made pontoon, which is home to 500 breeding common terns, was horrific.
“Basically, we suspect it was rats who completely destroyed this young colony. The scene was just horrific.
“Once they managed to get on to the island they virtually had an all-you-can-eat feast.
“Because there was so much for them to eat, they ate just the brains of many young terns, which is the most sought-after part.
“It was a fairly gruesome scene but it proves the dangers and threat which rats pose.”
The incident is the latest in a number of such attacks by rats on seabird colonies around Ireland, according to Mr Hatch.
He said that while nobody liked to carry out culls of any animals, it was necessary to keep the numbers of rats down.
He said there had been a similar problem on Dalkey Island and also with various other islands where puffins had been preyed upon in their nesting burrows.
“Rats are not native to Ireland and were probably brought here on ships from Asia. It is difficult to eradicate them as you have to lay poison that has dangers to other species.”