Inspectors with the Revenue Commissioners are investigating the discovery of green diesel in 15 vehicles on the Aran island of Inis Mór this week.
The misuse of marked gas oil was confirmed during checks on the largest of the Galway Bay islands on Thursday, a Revenue spokeswoman said.
Marked gas oil or green diesel, normally used in agricultural vehicles, attracts a low rate of excise. Its use in ordinary road vehicles is illegal. It is unusual to find so many vehicles with the illicit fuel in one inspection.
Revenue said its policy was to investigate all such detections with a view to prosecution.
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The maximum fine for a first offence on summary conviction is €5,000. Last year there were 310 convictions for such offences.
