A total of 122 unlicensed taxi drivers were prosecuted last year.
They either had no driver or vehicle licence, or allowed an unlicensed driver or vehicle to operate as a taxi.
According to details released under the Freedom of Information Act, the figures for 2019 are down from 178 the year before.
But Vinny Kearns, a former vice-president of the National Taxi Drivers Union, claims many illegal taxi drivers are escaping punishment.
"I would believe that they are not a true indication of the figures that are out there because we don't have enough enforcement officers to adequately police that.
"On a Friday or Saturday night, I believe there are quite a number of illegal taxis operating, drivers who don't have a licence or vehicles that aren't licenced."
In 2017, there were 122 prosecutions against fraudulent operators. This rose to 178 in 2018.