Yes, if the Garmin was loose on top of your jacket, and not in a dashboard mount, touching it was an offence. At least in the UK.
One of my friends is a prison officer in The 'Joy, from his stories of the Gardaà he deals with I had no illusions about their behaviour and am not shocked nor disappointed nor even surprised to hear your tale.
Before deciding whether you want to fight it, it might be worth reading all the papers they sent you carefully. Not only are they untrustworthy but they are also fairly incompetent and you might find something contradictory, or the wrong registration number, or something. Worth a careful look.