English: The Dodder rises on the northern slopes of Kippure in the Wicklow Mountains, and is formed from several streams. The headwaters flow from Kippure Ridge, and include, and are often mapped solely as, Tromanallison (Allison's Brook), which is then joined by Mareen's Brook, including the Cataract of the Brown Rowan, and then the combined flow meeting the Cot and Slade Brooks.

In the river's valley at Glenasmole are the two Bohernabreena Reservoirs, a major part of the Dublin water supply system.

The Dodder is 26 kilometres (16 mi) long. It passes the Dublin suburbs of Tallaght and then Firhouse, travels by Templeogue, passes Rathfarnham, Rathgar, Milltown, Clonskeagh, and Donnybrook, and goes through Ballsbridge and past Sandymount, before entering the Liffey near Ringsend, along with the Grand Canal, at Grand Canal Dock.

There is a weir just above the bridge at Ballsbridge and the river becomes tidal roughly where the bridge at Lansdowne Road crosses it. The Dodder and the River Tolka are Dublin's second largest rivers, after the Liffey.