I've driven the Miltary Road to the Sally Gap many many times but never really spotted Bohernabreena reservoir. I thought it was one valley over, but its not. At no point on the Miltary Road (without stopping) can you see it. Randomly one day I drove down the Garadhu Road and found this lovely little area. Now theres a cache there for you to find, and even on the busiest of bank holidays this place will be quiet and peaceful.
There is parking for three or four cars right at GZ.
Note: Its always a risk to leave your car in the mountains so I recommend you empty your car COMPLETELY before doing so. Take out your boot cover, open your glovebox and show the car is empty. However, GZ is right next to your car so you don't have to worry here, but I'd still not recommend doing this cache at night.
Note 2: This road is steep but is perfectly manageable in a car and fine in the summer. However, if its WINTER, pay attention to the traffic websites as you will NOT be able to get a car back up this road if its icy. Bear in mind the whole Military Road can be closed if its snowing heavily. You'll be fine 95% of the time but just keep an eye out in winter.