Nant Irfon lies high in the Cambrian Mountains on steep valley slopes near the source of the river Irfon, which joins the Wye further downstream. The valley owes its shape partly to river erosion and partly to past movements of ice. It is a typical example of upland habitats in this part of the country. The oak woodland, a remnant of the ancient forest which used to cover much more of Wales, together with the frequent rock exposures on the open hill, support a wide range of important lichens and mosses. The Reserve is a small part of the extensive Elenydd SSSI and the Elan Valley Woodlands SAC.
There is open access throughout the Reserve but there are no maintained paths. Extensive walking is possible although the terrain is generally rugged.There are steep and slippery slopes, deep pools and swiftly flowing streams.