Fraud squad detectives have been granted an extension to question the former chief of Anglo Irish Bank about alleged financial irregularities.
Sean FitzPatrick - former chairman and chief executive of the now nationalised bank - was arrested during a dawn raid at his luxury home in Greystones, Co Wicklow, south of Dublin.
Financial records were among documents removed from the house as part of a one-year investigation into the near collapse of the property lender and other Irish banking institutions.
The 61-year-old ex-banker stepped down in December 2008 when it emerged he had hidden loans worth €87m with Anglo Irish Bank.
The bank's headquarters at St Stephen's Green in central Dublin was raided by the Garda fraud squad and investigators from the Office of the Director of Corporate Enforcement last February, but Mr FitzPatrick is the first person to be arrested.
Detectives were granted an extra 12 hours to hold him after an initial 12-hour period elapsed.
In a brief statement, Finance Minister Brian Lenihan said: "I have always stated that there is an extensive garda investigation under way. I have been cautious not to prejudice that investigation and am eager to see justice take its course."
Earlier this month Anglo Irish Bank began legal action against Mr FitzPatrick to try to recover loans of €70m.
Former finance minister Alan Dukes, incoming head of state-run Anglo Irish Bank, has said the court case would be treated in the same way as any other debt.
Over a period of eight years up to 2007, Mr FitzPatrick temporarily transferred loans with Anglo Irish Bank to another bank before the annual report was audited.
It later emerged the loans reached as much as €129m at one point in 2007.
The Office of the Director of Corporate Enforcement and the Financial Regulator are also investigating whether Anglo Irish used more than €7bn of short-term deposits from Irish Life & Permanent to mask big customer deposit withdrawals.
The Irish Government has already pumped €4bn euro into Anglo Irish since it was bailed out last January.
Expected record losses to be announced later this month could see the taxpayer being asked to inject further cash into the nationalised lender.
Anglo Irish is also expected to transfer around €28bn of loans to the state's "bad bank", the National Asset Management Agency.
© Press Association