The Revenue Commissioners are investigating 3,600 Irish firms and individuals who have bank accounts in Switzerland and Liechtenstein to make sure tax has been paid on the money.
Some of this group have recently received letters from the Revenue challenging them on where the money originated from. "This is an ongoing investigation,'' said the Revenue yesterday.
The organisation, which published its annual report, got a tip-off from another country about bank accounts in Switzerland held by Irish residents, but declined to name the country.
However, last year it was revealed that the German government paid money to buy a computer disc with the names of thousands of people with secret Swiss bank accounts.
All the Irish Revenue would say yesterday was its information did not come from Switzerland itself.
The revelations of a probe into Swiss bank accounts came as the Revenue warned that growth in the "shadow economy'' was likely due to the recession. "The biggest risk in the so-called shadow economy is suppression of sales and under-declaration of income,'' said Revenue chairwoman Josephine Feehily.
She said the Swiss investigation was going on intensively at present. "We have more checking to do before we can say whether those dealings cause us concern or not,'' she said.
The Revenue have been hitting Irish banks with court orders to force them to give the Revenue information about transfers between Irish banks and those in Switzerland and Liechtenstein. It is understood some accounts in Jersey and Guernsey are also being examined.
The Revenue made it clear that some of the people involved have already made a disclosure to them in 2004, but others have not and they have received letters.
The Revenue Commissioners also revealed yesterday they were continuing another probe into 300 directors and executives from the banking sector.
To date, €1.3m has been collected on the back of these enquiries, although Ms Feehily played down the scale of activity being investigated, saying much of it related to benefit in kind.
The Revenue are also working closely with NAMA and have collected information on 108 cases. Ms Feehily said what Revenue wanted to know was how certain developers and borrowers financed certain deals and where they got the money to do this. She said her organisation had a very good relationship with NAMA, which is led by former Revenue chairman Frank Daly.
Meanwhile, it has emerged that over 15,000 people and firms can only pay their tax debts by instalment due to the severity of the downturn, the Revenue revealed yesterday.
The Revenue said it had put "payment arrangements'' in place for individuals and firms who owe €160m. But it warned that only those who come forward early will get such arrangements.
The Revenue is not meant to be a "lender of last resort'', said the organisation.
During the year the Revenue collected €31.92bn, just 2.25pc ahead of the Budget estimate. Thirteen court convictions for serious tax and duty evasion were taken in 2010.
Audits by the Revenue produced €434m in the year, while other assurance checks, which are less severe than an audit, produced proceeds of €58m.
A senior official has, meanwhile, been appointed to assess any implications from the Moriarty Tribunal report, said Ms Feehily.