Ballydonnell Brook is a short whitewater run located in County Wicklow, Leinster, in the upper River Liffey catchment. The run is graded 3 to 4 and covers approximately 2 kilometres, making it a compact but technically demanding paddle.

The brook drains a large upland valley and flows northward to join the main course of the upper River Liffey between Ballysmuttan Bridge and Ballylow Bridge. The floodplain sits at an elevation of around 300 metres above sea level. The channel itself is cut 2 to 5 metres into the valley floor, which is composed of glacial tills and moraines containing granite boulders, gravel, and sandy material. The streambed features granite boulders and cobble gravel bars, which contribute to the technical character of the run at this grade. A notable confluence occurs where Ballydonnell Brook meets Luqaculleen Brook, a tributary joining from the left.

The surrounding area is remote and accessed via forest tracks. Given the elevation and the nature of the upper Liffey catchment, paddlers should plan accordingly for a committing approach. The brook is part of a geologically significant post-glacial fluvial landscape, and the valley retains a largely natural character shaped by thousands of years of river activity following the last glaciation.