GARDAI have issued a warning to retailers over a flood of high-quality forged €20 notes.
The counterfeits have been found in Dublin, Limerick, Cork, Waterford, Tipperary and Kildare.
It is feared that the forgers are using music festivals and other major events to distribute them.
By using €20 rather than €50 notes, the crooks are making it more difficult for businesses to detect them given the sheer volume of that denomination handled each day.
While €50 notes are routinely scanned with UV swipe technology, €20 notes are rarely checked.
Unlike most forgeries, the fake €20 notes now in circulation use high-quality paper very similar to real cotton-based notes, as well as a hi-tech printing system that makes them very difficult to distinguish from the genuine article.
The discoveries came only months after gardai smashed a forgery network run by a Dublin-based gang and seized almost €2m worth of forged notes.
Officers also found two printing presses that had been imported from Europe along with specific forging technology.
Detectives urged retailers to carefully check euro notes using the usual safety features including raised print, micro-printing, see-through numbering, a watermark, security strip and glossy stripe.
The forgeries are being passed in transactions ranging from €40 to more than €200.
One retailer was recently caught for €240 when fake notes were used to buy electronic goods.
Gardai have now urged retailers to carefully check all currency and use 'swipe' scanning technology if it is available.
"The rule is to check all cash that is passed whether it's €20, €50 or €100 notes," a spokesman said.
"Swipe technology is a great help, but even high-quality forgeries can be detected with the naked eye in a careful examination."
Forgers have traditionally focused on larger-denomination banknotes, but the scale of the problem became so severe that many retailers declined to accept notes of €100 and above.
Major efforts have been made to crack down on forgery involving €50 notes with special verification tests becoming widely available.
Counterfeiters on the Continent switched their focus to lower denomination notes such as €10 and €20.
While the profit margins are lower, such notes are easier to pass off because so many are used for most transactions.
The dramatic increase in the quality of forged notes is attributed to the emergence of the latest generation of hi-tech printers that can closely replicate notes issued by the European Central Bank.
Gardai stressed that forgers have not defeated the simple verification methods used by retailers.