Climbed Knockbrack and Dromderalough on 13/11/05 which was a bright but cold day.
The minor road on the right is on the Kenmare side of Kilgarvan, just after the sign for the GAA grounds but before the Michael J Quill Memorial Centre. There is a right turn roadsign about 100m before the minor road.
As it was such a fine day, I decided to park just south of Mangerton Bridge at V995762 B using a good layby on the right. I then followed the road north, then west to the last farm at the end on my right. Here, you will see the rather intimidating Private Property signs as mentioned by mat in his comments on Dromderalough. I counted 3 of these signs, but a 4x4 vehicle passed me at the entrance to the forest and I was not challenged.
I would suggest following the route up Knockbrack as described by Grumbler#1 in his comments on this mountain. Paddy Dillon in his book The Mountains of Ireland suggests leaving the track at the forest entrance and following the forest boundary fence in a north-westerly direction. Staying on the forest track is an easier option, although perhaps slightly further. Look out for the sheep fence on the final approach to Knockbrack, complete with a barbed-wire top. Views from the top are good, with the Reeks, Stumpa Duloigh and Mullaghanattin all prominent. I encountered a small group of red deer just north of the summit on the way to Dromderalough.
Dromderalough appears to have 3 separate summits, all marked by cairns. I continued on to Mangerton, which involves crossing some very rough and wet bog. This was hard going, and adds around 2 hours to the walk if you return to Dromderalough from Mangerton. On top of Mangerton, there was a group of at least 30 school children. On returning to Dromderalough, I descended south-easterly across open hillside to return to the entrance to the forest. Here, you can rejoin the forest track next to some sheep pens. As I walked past the last farm on the way out, I met the farmer. He told me the signs had been put there by the company (South West Forestry) which manages the forest. Parking in the forest is possible, but may not be a good idea in view of the private property signs. The road walk to and from Mangerton Bridge adds around 4km in total, but serves as a good warm up/warm down. Parking here is also not obstructive in any way.
Knockbrack and Dromderalough may not be the most spectacular hills in the area, but are well worth the effort. Linkback: https://mountainviews.ie/summit/255/comment/2055/