Phnom Khieu, Green Mountain (literally, Blue Mountain, but blue and green are used interchangeably in Khmer) is just east of Pailin city along National Highway 57.

The waterfalls, where many Cambodian visitors are going, is indicated by a large billboard on the south side of the highway. Then you're not there yet. There is a gravel access road, when you have taken the road indicated by the sign. If you're there at the waterfall, they ask a small fee per visitor and per vehicle.

The dense mountain range Phnom Khieu is part of perhaps the last major piece of forest around Pailin. Almost all of the land along the road was cleared of forest in the three years since my last visit to Pailin. Usually there are now cassava plantations in place of the forest. 
It is about a kilometer over an ever steeper and twistier road up the mountain until you'll reach the entrance of the attraction. 
Nowadays almost all the particular birds are gone along this wooded path. The road goes down and you have to cross two streams. When you arrive at seven small waterfalls, wyou'll see a restaurant and a series of wooden sheds. From here you can reach the waterfall.

A tempting footpath leads to the side of the mountain past the waterfall. The locals say that this path also leads to a number of artisan gemstone mines. We took this road for about a kilometer up the mountain before returning. 
I would not recommend to take this path, because there is a high possibilty that there are landmines in the area. 
This region was headed by Leng Sary's department of the Khmer Rouge. This lasted until the mid-1990s. The landmines were used to scare people off, if they wanted to reach the gem mines. I doubt very much that all mines have been removed. 
If anyone wants to go this path, I highly recommend caution to leave the rocky path, for whatever reason.