Police said that shortly before midnight they raided a house in the inner north Dublin district of Phibsborough and stopped a car on a highway ringing Dublin. A third of the stolen money has been recovered.
Sgt. Alan Roughneen said five men and a woman were arrested in Phibsborough, and one man was arrested in the car. Authorities also seized six cars, checking to see if they were used to move hostages or money.
On Friday, six armed, masked men stormed into the rural home of Bank of Ireland worker Shane Travers. They tied up his partner, her 5-year-old son and her mother, and told Travers they would be killed unless he cooperated.
Such hostage-taking tactics are common in Ireland's criminal underworld but never in Republic of Ireland history have they netted anything close to the $9 million that Travers carried out from his branch Friday morning.
His family had been abandoned inside a van north of Dublin, but escaped on their own and were not seriously harmed.
Initially, Justice Minister Dermot Ahern and police chiefs offered veiled criticism of Travers' apparent failure to notify police until after he had handed over the mountain of cash. That violated police and bank instructions on how to handle a bank robbery involving hostage-taking.
So-called "tiger kidnappings" when gangs seize families of bank officials and force them to breach their employers' security are common crimes in Ireland, a close-knit society where criminals can closely track their targets. But they typically involve thefts below $1 million.
Friday's raid on the Bank of Ireland branch in College Green, the tourist heart of Dublin, represented by far the biggest robbery in the history of the Republic of Ireland. But it pales in comparison with a similar 2004 raid in the neighboring British territory of Northern Ireland, when two Northern Bank employees were forced to help a gang take more than 26 million British pounds from the bank's central Belfast vault.