Garda will not be interested, no crime is committed, as far as I know clocking is not against the law, somebody would have to purchase the car and then follow up for the Garda to be interested.
Is this an SIMI garage, if so report it to the SIMI, I am not saying much will happen, you are in a tough place on this one, look for another job, these are not good people, clocking is a common practice, may of the main dealers have guys coming in at night to alter the odometer via laptop when the garage is quiet, at your place they don't bother trying to conceal it, there have been high profile convictions in the past.