A taxi driver ripped off passengers 280 times by secretly using a remote control to add €9 to fares without their knowledge, a court has heard.

Dublin District Court heard today that the scam unravelled when a tip-off led to one driver, followed by a recall of 206 other taximeters with which the remote device was compatible.

Judge Anthony Halpin said the amount of over-charging could have been "colossal".

Robert Griffin, 66, of Maplewood Park, Springfield in Tallaght, was fined €750 after he pleaded guilty to 280 counts of over-charging contrary to the Taxi Regulation Act.

He was accused of using a concealed remote control device 20 times a week from 4 February until 10 May this year.

He was prosecuted following an investigation by the National Transport Authority (NTA) which discovered he covertly topped up the final fare on the taximeters.

The increase was added to the final fare on the completion of passengers' journeys.

NTA inspector Liam Kavanagh told Judge Halpin that earlier this year he received information that a number of taxis were over-charging passengers using the remote device, which was linked to some meters.

A confidential source led him to a taxi rank at Tallaght University Hospital where the defendant was sitting in his car, he said.

Mr Kavanagh told the court he carried out a routine inspection and found the remote control device in the driver's door.

He suspected an offence had been committed and advised Griffin to hand it over.

"He admitted he had used the remote control to add extras to taxi fares," the inspector said.

He was "visibly shaken" and told the NTA official he was not well.

Mr Kavanagh told Judge Halpin he did not want to cause the driver too much upset at the rank and told him they could meet at the NTA inspection centre the following day, "and leave it until emotions had calmed down".

Griffin agreed and when he went there his meter was inspected by the taxi metrology service. He was cautioned by the inspector and made full admissions that he used the remote device to add to fares.

Mr Kavanagh explained that Griffin kept the remote fixed to the driver's door with velcro and as he drove along he pressed it, increasing the fare by €9 per journey.

The court heard some other drivers who used the device had kept it between their legs.

"The scam, for want of a better word, was closed down," Mr Kavanagh said.

"The mind boggles," remarked Judge Halpin after the facts were presented.

The NTA identified 206 taximeters that were sensitive to the remote, the court was told. They have been recalled for new software.

Not all of the recalled meters were linked to criminal offences but, the inspector added, they were all, "sensitive to this device".

In pleas for leniency, the defence said Griffin, a grandfather, had heart issues and other health problems that were of grave concern. He did not give evidence or address the court at his hearing today.

Griffin offered a full apology and accepted full responsibility, the solicitor submitted.

The offence carried a maximum possible fine of €4,000 and there were 280 counts on the summon, the court was told.

Finalising the case, Judge Halpin said the aggravating feature was the over-charging of people and use of the remote to bypass the proper measurement of fares by the taximeter.

Fining him €750, Judge Halpin noted Griffin's early guilty plea and he accepted these proceedings gave rise to some of his health problems.