There was a 46% increase in the number of unlicenced 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 did not have the driver or vehicle licences required.
Vinny Kearns, a former vice-president of the National Taxi Drivers Union, claims many more illegal taxi drivers are escaping punishment.
"It's good to see an increase in the detections but I believe it's only the tip of the iceberg," said Mr Kearns.
"If you look at activity over the weekends, I believe that every single weekend there are a substantial number of taxis operating that are neither licenced as in the vehicle or the driver driving it isn't licenced to drive the vehicle."