write in immediately to the superintendent in charge of the garda involved at whatever station or district. If its a genuine error in reg, etc. it will be cancelled. If not, it will be at least be thoroughly checked out why it was issued in first place. The garda will have to account to his superintendent why and how the notice was issued if the car is not in use. Give it a few days and check on what happens as Garda could have taken wrong details, etc. - it can happen computers or no. If the car was not actually on the road at all, sounds more like a garda error which will have to be cancelled.