Put Killarney Glen into google maps. Maybe it won't be in your cars GPS, but Google has you covered. The start of the walk is just off Beechmont Road. You can usually spot the start of the walk by the cars parked on the side of the road when the car park is full. If you are driving from the north you will see a sharp switchback in the road and the car park is just before that. There is only a small sign for Killarney Glen, so it is easy to miss it.
Another landmark to look for is Marian Valley Shrine. There are two access roads to Marian Valley. If you are driving from the north they will be on your right. After you pass the first access road on your right you will the entry to Killarney Glen on your left. If you go past the second access road to Marian Valley, then you have gone too far.