It is culverted for its upper course, visible in St Anne's Park for its entire lower course, and causes flooding somewhere along its line most years.

The origins of the name, also spelt sometimes as Nannikin or Naneken, are unclear, though it has been speculated that it might be a diminutive reflection of the much-larger Nanny River flowing just north of the County Dublin boundary.

It passes under Schoolhouse Lane and the Oak Park development, then by siphon below the Dublin Port Tunnel's northern portal area.

The river passes under Howth Road at the site of Ballyhoy Bridge, and emerges from its culvert to flow openly through St Anne's Park.

It passes in a shady channel through an area of playing fields, runs through the Dublin City Millennium Arboretum, and then flows in a more distinct valley through the central reaches of the park.