O'Brien's bond sale, on 22 February, which was engineered to fund his buyout of the minority owners of the Caribbean mobile phone company, saw him increase his estimated wealth by two-thirds to about $2bn.

The Sunday Times put O'Brien's wealth at Euro904m in April 2006. Gains in his share portfolio, including subsequent acquisitions of a substantial minority stake in Independent News &Media, would have increased it modestly since then. But the Digicel cash places him firmly in the billionaires' ranks.

O'Brien's estimated wealth of $2bn would make him about the richest person in the world, according to the Forbes list, placing him above rival IN&M chief executive - and corporate rival - Tony O'Reilly, who comes in at $1.7bn.

Other Irish billionaires who made the list were Se�n Quinn at $4.5bn, John Dorrance at $2.6bn, and Dermot Desmond at $1.8bn.

O'Brien had originally planned to take Digicel public on the New York stock exchange, but when two private equity bids valued the company at $3.8bn, he decided to make use of his 78% stake instead.

"I saw how much debt they planned to put on and thought, 'Why not do this myself?'" he told Forbes. "I said, 'Pull the trigger'. We did a seven- or eight-day road show. Bang."

His instincts have proven to be shrewd. Four days after the bond sale, the stock market began its slide, knocking nearly four percentage points off its value in a week. An IPO under the circumstances probably wouldn't have earned O'Brien as much, especially since Digicel's $2.8bn debt stands at 8.5 times putative earnings before interest, tax and depreciation.

Having made his bundle, though, O'Brien has pronounced the widespread availability of cheap financing "not sustainable".