Almost €4.5m has been stolen in invoice redirect frauds since the start of the year, according to gardaí.
More than €2m has been stolen in the last month alone, they sad.
Under the fraud criminals send emails to businesses purporting to be one of their legitimate suppliers with the supplier's new bank account details and a request to change the account to one that will ultimately benefit the criminals.
These requests can also come by way of letter or phone call.
The Garda National Economic Crime Bureau says various companies including sports clubs, accountants, hospitals, car dealers and pubs have redirected payments to fraudulent accounts solely on receipt of an email.
The bureau says it has recovered more than €1.8m, but has once again warned businesses not to rely on emails to change payment details and verify the request before making payments
Last year Detective Chief Superintendent Patrick Lordan of the Economic Crime Bureau said this type of fraud had been going on for some years.
He said the criminals use emails that look like the real suppliers' address but with some slight difference.
Often the fraudsters use students' and young people's bank account details by offering them a fee to allow use of their accounts for a few days.
Earlier this year an investigation was launched by the bureau into these so-called money mule accounts.
According to the Garda National Economic Crime Bureau, there has been a dramatic and worrying increase in CEO and invoice redirection fraud in the past few months.
Since January 132 cases have been reported with a total of over €4.4m stolen, €1.28m has been recovered but €3.2m is still missing.
Gardaí say the thieves succeed because while the person who makes the payments in a company is a senior manager, the person who can change the bank details for payments is often a junior clerk.
The fraudsters are also credible, they often create a false sense of urgency and the businesses do not verify the criminals' details before handing over the money.