Update 2.54pm: The Irish Taxi Drivers Federation has expressed surprise at the high rate of arrests during an illegal immigration crackdown.
It has emerged gardaí targeted cab drivers in May after noticing an unusual pattern in applications for public service vehicles.
Out of 110 stopped, around a quarter were suspected of being married to a European woman they'd never met for the purposes of living here.
The federation's president Joe Heron says it's difficult to get a taxi license.
"I was very much surprised and what really surprises me is there were so many non-nationals able to get PSV drivers licences," he said.
"They must have been in the country for a considerable length of time before they were even stopped.
"The geography part of the test, the knowledge of the area in which you're working, it should take you a considerable length [of time] to know the city to pass it [the test for a PSV licence].
"The first thing you do is apply at your local garda station and they put you through a vetting process. I don't know why these things didn't come up at that stage, about how these people are in the country.
"You will have to sit two parts of an exam.
"One is an industry knowledge part of the exam. The other is the area knowledge for whatever area you want to drive a taxi in.
"If you take Dublin, lots of people who were born and reared in Dublin have failed the exam."
As many as one in four taxi drivers stopped at recent illegal immigration checkpoints are suspected of being in a sham marriage.
According to the Irish Times, an operation was put in place to coincide with Ed Sheeran's Dublin concerts in May.
Of the 110 drivers stopped, 26 are believed to have paid to marry a woman from Europe so they could have the right to work in Ireland.
Gardaí have revoked their immigration status, which is now under appeal.
The 110 non-EU drivers were stopped across three checkpoints during the one-hour operation, led by the Garda National Immigration Bureau.
Taxi drivers were targeted due to unusual patterns detected in new public service vehicle (PSV) applications.
Last January, of the 58 new applications for PSV licences, 40 were made by non-EU nationals.