This includes Dublin Zoo, which was hit last December by a scam costing it around €500,000.
The Garda National Economic Crime Bureau (GNECB) has managed to freeze in excess of €300,000 in bank accounts used by the criminal outfit.
The operation is one of up to 40 investigations the Money Laundering Unit of the GNECB is conducting into what is known as Invoice Redirect Fraud.
The scams have ranged from as little as €7,000 to as high as €4.3m that amount relating to a fraud affecting Meath County Council in December 2016.
In the fraud, criminals purport to be a legitimate supplier of a business or organisation and instruct them to send payments to a new bank account. When the business targeted does so, the criminals transfer the monies into a network of accounts and then proceed to cash the money.
In the latest operation, the GNECB conducted a number of searches in Tipperary and Dublin last week. They were assisted by computer specialists from the Garda Cyber Crime Bureau. They arrested one woman in Blanchardstown, west Dublin, and four people — three men and one woman — in Clonmel, Co Tipperary.
They range in age from their 20s to 60s and includes an Irish national and four foreign nationals, mainly eastern European, and one possible Pakistani individual. Gardaí are endeavouring to establish their identities and piece together how they know each other.
They were detained under Section 4 of the Criminal Justice Act 1984 in Clonmel and Cahir stations in Tipperary and Tallaght in Dublin.
The provisions allow for 24 hours in custody. All five were released late on Wednesday night.
The GNECB is now preparing a file in relation to four of the people and is expecting to seek directions from the DPP to bring charges in relation to both money laundering and theft.
Charges of money-laundering are more difficult to secure, but gardaí said they are now bringing more such charges.
The GNECB succeeded in freezing a “substantial amount of money” — believed to be in excess of €300,000.
Sources said invoice redirect fraud is typically highly organised, but did not necessarily involve significant financial knowledge.
One person often sets up the bogus account and a second person engages in the fraud and transfers the money into other accounts while a third person cashes out the monies.
Two companies were targeted in this investigation, one being Dublin Zoo.
It is understood that the GNECB and its Money Laundering Unit, have or are investigating up to 40 invoice redirect frauds in the last year and a half. Gardaí said criminals typically contact businesses by email but also do so by letter or phone.
The GNECB has urged businesses to have “robust policies and procedures” in place, including having staff consult with a supervisor when requests regarding a change of bank account comes in from a supplier.
Staff are also advised to make “direct contact” by phone with someone they know and trust in the supplier to verify the request and if they are beginning business with a new supplier.