Giant's Causeway stones sold for £19,000
The seven two-tonne basalt stones once belonged to Rathmore Golf Club in Portrush, home course of US Open winner and Ryder Cup hero Graeme McDowell.
They were bought by the club in the 1970s and put in the car park as a bulwark against possible damage in car bomb attacks.
They were bought by an overseas phone bidder.
James Rylands, director of Summers Place Auctions, said the stones were "a real piece of history".
"We've never had anything like this before. It's incredibly rare," he said.
"We have heard that some people may have carried away stones from the Giant's Causeway before, but nothing on this scale.
"There are very few other locations in the UK, or indeed throughout the world, where there are similar geologically configured stones like these."
The Giant's Causeway, Northern Ireland's top tourist attraction with 750,000 visitors a year, was declared a World Heritage site by Unesco in 1986. It has been owned by the National Trust since 1961. A new 18.5 million visitors' centre is due to open in 2012.
A Trust spokeswoman said the removal of stones from the site would be in breach of conservation regulations.
She said: "Naturally, as custodians of the Giant's Causeway World Heritage site, we are disappointed to see basaltic columns for auction.
"That said, we cannot actually prove they are from the World Heritage site, or if, or when, they would have been from the Giant's Causeway."
Rathmore Golf Club sold the stones last year for a nominal fee to a keen gardener who sold them on to a man in England.
It is thought the stones may have been taken from the world-famous tourist attraction on the north coast of Northern Ireland several decades ago.