I don't know what the story is in this case, but from my experience in my job any measuring device which records something as required by law or other standard e.g. ISO has to be properly calibrated, with the folowing requirements.
1. Must be calibrated within last 12 months by a properly licenced independant company
2. A label must be placed on device stating when last calibrated, when due for next calibration etc
3. A proper certification must be issued stating the measurements found against the standard, the serial number of the device etc.
4. The calibration certificate for the machine that was used to calibrate the device should be have been checked and a copy held on file
If points 1 & 3 above cannot be satisfactorily shown, then the device cannot be proven as calibrated. As for 2 & 4, these are not necessarily required by law but is required by many standards e.g. ISO
Though, I doubt a Garda would have all the above paperwork in his/her car. If I was in the situation, I would take the serial number of the speed camera and ask for a copy of the calibration certificate if he didn't have it. Though, having said that I would only challenge it if I thought I was being unfairly treated. If I knew I was well over the speed limit, I wouldn't complain