The Chairman of the Irish Nationwide Building Society, Dr Michael Walsh, resigned this evening.
The Society has been at the centre of questionable loan arrangements involving the former Chairman of the now nationalised Anglo Irish Bank.
Earlier this week, Moody's downgraded Irish Nationwide's debt ratings, warning it would have to change its business model by cutting its large exposure to commercial property.
The decision had been unexpected by the Society.
Both the Government and the Financial Regulator have been made aware of Mr Walsh's resignation.
The choice of Dr Walsh's successor is a decision for the board of Irish Nationwide.
There are two public interest representatives on the Society's board arising from the State guarantee introduced last September.
The new chairman will have to be approved by the Financial Regulator.
As well as being Irish Nationwide Chairman, Dr Walsh is a director of number of other companies.