Ernst & Young is being sued by the former Anglo Irish Bank for its conduct leading up to the bank's collapse in 2009, in what Irish media is calling a "watershed moment".
This is the first time an Irish bank has ever sued an auditor over its role in the financial crisis.
The former Anglo Irish Bank, which is now Irish Bank Resolution Corporation (IBRS), said in a website statement that proceedings had been started against E&Y relating to "the role of Ernst & Young as auditors to Anglo Irish Bank pre-nationalisation," but said it would be inappropriate to comment further.
E&Y said it was aware of the proceedings, but also refused to elaborate.
In 2009, the bank was nationalised in the wake of the financial and property crash.
The rescue almost bankrupted Ireland, costing the country €23.9bn that was widely used by property developers to keep the country's housing market afloat.