Augh or Agh in a placename is generally thought to come from one of three sources, depending on the situation of the name: Áth – a ford, Achadh – a field or Each – a horse.
However along the slopes of Blackstairs Mountain and White Mountain and adjoining districts, it is clear that there are many water features associated with the prefix Augh. Going by convention you'd think that they refer to fords, so Aughnagappull below may well come from Áth na gCapaill, ford of the horses. However the large number of such names and nature of the qualifiers in other examples, lead one to conclude that Augh or Agh in this area signifies a stream, usually a mountain stream and perhaps equivalent to Sruth in other parts. So Aughsollagh is the muddy stream and Aughavay, the stream of the birch trees etc. The term Augh here may be related to 'abh' or 'abha', pronouncd 'aw' and 'ow' respectively, a river.
I've enquired with local people as to understanding of the name with no definitive proof to date. I've heard it usually pronounced as in Augh – Ach but also as Auk, as in the bird. A local history guide to Killanne and Rathnure gives 'Augh na Daragh i.e. streamlet of the oaks'. Here's a list of water features where the prefix Augh/ Agh applies in this region.
Augh an Uisce
Augh na Daragh