Seldom seen in Irish waters, the small crabs attached themselves to goose barnacles clinging to the barrel and were found on Kilcummin Strand in the Castlegregory area of Co Kerry.
Normally resident in the western Atlantic, mainly in the Sargasso Sea and the Bermudas, they are believed by marine expert Kevin Flannery to have floated thousands of miles across the ocean.
“They live near the surface of the ocean where there’s a lot of weed to feed on. They like warmer waters and cling to things that are floating in the sea and sometimes to turtles,” he said.
The last pair to be found in the Dingle area, about a decade ago, were stuck to a turtle. They can also fasten themselves to buoys, ropes or pieces of wreckage.
The barrel washed up on Kilcummin Strand was covered by large barnacles. The crabs feed on small organisms living around the barnacles.
Columbus crabs are rare in European waters. The main population appears to be in the western Atlantic, from the eastern US to Uruguay but especially in the Bermuda Triangle, in the Sargasso Sea.
The latest finds are going on display at Dingle Oceanworld, an aquarium which attracts 100,000 visitors annually.
In recent years, large numbers of exotic fish have been found off the south-west coast, but numbers have declined in the past two years, according to Mr Flannery.
“There seems to have been a downturn in the warmer currents we had been experiencing, with less fish coming up from the southern seas,” he said.
The Columbus crab, meanwhile, is also known as the Gulfweed crab and is reputed to have been first found by Christopher Columbus on his voyages of discovery. It is a small crab with a rectangular shell about half an inch across, with large claws for its size and hairy fringes to its legs to help it swim.