There was a 46% increase in the number of unlicensed taxi-drivers last year.
In 2017, there were 122 prosecutions against fraudulent operators but this rose to 178 in 2018.
According to details released under Freedom of Information, most offenders didn't have the driver or vehicle licenses required.
Vinny Kearns, former Vice-President of the National Taxi Drivers Union, claims many more illegal taxi-men are escaping punishment.
He says it's good to see an increase in detection rates but at weekends he believes many extra unlicensed drivers are working around the country.