RTÉ News has learned that Irish Nationwide directly received a cut of profits from successful developments that it had financed.
The building society enjoyed increases in profits during the boom that surprised many in the banking industry.
Profits rose by an impressive 63% to almost €400m in 2007 before a tumbling with the property crash.
Despite the fact it was established as a building society lending home loans, four fifths of its revenue came from financing developers, including big names such as Liam Carroll and Ballymore Homes.
But RTÉ News had learned that not only was it lending to developers, but the building society was also directly profiting from successful projects.
Irish Nationwide set as a condition on certain loans to developers that the building society would get a proportion of profits.
This would usually apply when profits in a property project reached a certain point sometimes following a sale.
A percentage of the profits would be paid to the building society.
This could be in addition to the normal way a lender makes a return from fees and interest.
Financial sources have told RTÉ News the practice is unusual but not illegal.
The proposed National Asset Management Agency has been made aware of the issue. It is expected to take over more than half of Irish Nationwide's loans.
The disclosure comes after controversy earlier this week about the building society's former chief executive Michael Fingleton.
He has refused to repay a €1m bonus that he had earlier stated he would return to the building society following political pressure.
Mr Fingleton is refusing to repay until he sees a report on his pay and his €27m pension.
