My advise to you is to report it, even if you can't get the reg number they might be able to do something about it!
Something similiar happened to me two years ago. A neighbour crashed into my car while it was parked outside my house and drove off. I didnt see the damage until later that day after being to a supermarket so had presumed it was some idiot in the supermarket carpark. I was looking at the damage outside my house later that day when one of the other neighbours approached me and told me they saw everything that happened. I contacted the local Garda station and they looked at the damage etc (cracked bumper and it was gaping out from the car) and they called up to the neighbour who witnessed it who gave a full story. However, the neighbour who did the damage denied ever crashing into my car to the guards. The garda then told him that they would have to investigate it as a "hit and run" and then suddenly he confessed. It took well over 3 weeks for him to pay for the damage. Luckily, he moved away soon afterwards.
Good luck!! Hope you find them. Unfortunately I didn't catch the latest person to reverse into the side of my car