It is about 11 kilometres (6.8 mi) long;[1] it passes the nature reserve of Knocksink Wood and the village of Enniskerry, and joins the River Dargle near Bray.

[3] The Glendoo / Glencullen combined valley structure is formed along a fault, underlain with granite (Devonian period) to the northwest and schist (Ordovician) to the southeast, with glacial till and glaciofluvial gravels covering the bedrock.

[5] The rapidly-flowing river holds brown trout, as well as a modest presence of salmon in its lowest reaches, and some eels.

The wood is an official nature reserve, as "a woodland ecosystem which is of scientific interest"[7] and also a Special Area of Conservation.

Major features include petrifying water flows (tufa springs and seeps) and "old sessile oak woods", along with alluvial forest.