Announcing the nationalisation of Anglo Irish last Thursday night, Minister for Finance Brian Lenihan said the disclosure last month that bank chairman Sean FitzPatrick had hidden loans borrowed from Anglo over many years had weakened the funding of the bank.

Remarkably, for a bank that had long been under scrutiny for the rapid growth of its loan book, FitzPatrick had been treated with deference by the financial regulators here.

But experts from Irish universities say the role of another leading Irish businessman, Sean Quinn, once Ireland's richest man and owner of two regulated insurance companies, still remains hidden from the Irish public.

The amount of loans, if any, Quinn has borrowed from Anglo Irish in the past and the full details of his family's large shareholding in the bank remain hidden despite the Irish state, since September, acting as the guarantor of bond debt and customer deposits on Anglo's balance sheet.

It had long been speculated that Quinn had built up a significant indirect stake in Anglo Irish Bank over an unknown period through stock market bets, called contracts for difference (CFDs). Quinn had staked huge sums that Anglo's shares would continue to rise, but it was only in July that the Irish public were told how badly the bets had soured.

Quinn faced losses of about €900m on the CFD losses, and, in an attempt to stem further losses, he was forced to buy outright a 15% stake in Anglo, that exposed him to further losses.

The role of the Financial Regulator and its talks with Quinn about his Anglo shareholding have never been disclosed.

Despite calls by the Irish Association of Investment Managers (IAIM) for full disclosure, the Irish Stock Exchange refused to oblige Quinn officially to report his stake because, according to his spokesman, unnamed Quinn family members held the shares between them.

The Financial Regulator should have known at a very early stage that Quinn was building - indirectly through CFDs - a huge exposure in Anglo Irish shares because the code of the Bank for International Settlements, the adviser to central banks, require bank regulators to be aware of such things.

In March, Anglo Irish executives blamed a one-day 19 per cent collapse in the Anglo share price on unidentified short-sellers. The Financial Regulator subsequently stopped short-selling in Anglo and other Irish bank stocks and the ban remains in place.

In October, Quinn was fined by the Financial Regulator and resigned from the board of his insurance company because he broke rules by borrowing money from the regulated insurance unit to help him buy shares, including in Anglo Irish.

"We still do not know the truth about what happened between Anglo Irish and Quinn Insurance," said Brian Lucey, professor of finance at Trinity College Dublin.

The Financial Regulator has refused to answer questions on why it let Quinn build up a substantial stake in recent years through contracts for difference that he bet on Anglo's shares continuing to climb.

Elaine Hutson of the UCD School of Business said the public should be told the full story of what went on at Anglo Irish.

"I'd be very concerned that we know very little about what was going on despite us being the guarantors of the bank through the sovereign debt," she said

A spokesman for Sean Quinn said on Friday that he had no comment to make on Anglo Irish.