The River Liffey (Irish: An Life, meaning "River of Life") rises at Kippure near Sally Gap in County Wicklow at an elevation of around 500 metres and flows approximately 132 kilometres eastward through Counties Wicklow, Kildare, and Dublin before emptying into Dublin Bay on the Irish Sea. Its drainage basin covers 1,256 square kilometres. Average discharge varies considerably through the year, from around 3.7 m³/s in August to nearly 30 m³/s in December, making water levels highly seasonal.

The river offers a range of paddling experiences depending on the section chosen. The upper and middle reaches through Kildare and into west Dublin are graded roughly 2–3 and include a number of weirs and rapids. Key put-ins include Straffan, Temple Mills, Salmon Leap in Leixlip, the Sluice near Lucan, and points around Lucan village itself. The Sluice is a well-known feature: the recommended line is far river left, keeping the boat pointing downstream and paddling fast, with a deep plunge pool and an island below requiring attention. Islandbridge Weir, the final weir on the river, stands around 11 feet and can be shot on the right-hand end. Water levels are strongly influenced by dam releases, and the site liffeyreleases.com is the primary resource for tracking release schedules and volumes.

Several hazards deserve attention. The Sluice and surrounding area have a documented association with Weil's Disease (Leptospirosis), and paddlers should take appropriate precautions. Weirs that are benign at normal levels can become dangerous during floods. Fallen trees, particularly in the wooded Strawberry Beds section near Lucan, can occasionally block the river. Car break-ins have been reported at the Sluice car park and other access points around Dublin; valuables should be locked out of sight.

The lower tidal section through Dublin city is a different experience entirely, navigable by kayak at appropriate tide stages. Paddlers pass landmarks including the Custom House, the Ha'penny Bridge, O'Connell Bridge, and the Samuel Beckett Bridge. City Kayaking operates guided 90-minute tours from a pontoon on Liffey Boardwalk at Bachelor's Walk, timed to the tides, using sit-on-top doubles and triples suitable for paddlers aged 8 and up.

The annual International Liffey Descent race, organised by Canoeing Ireland, is a significant event on the river and draws competitors through the whitewater sections from Straffan toward Dublin. Access to some put-in locations, including Salmon Leap, may be restricted outside of organised events as the clubhouse there is privately operated.