|- location||Anglesborough, County Tipperary|
|Celtic Sea at Waterford Harbour via River Suir|
|Basin size||99 km2 (38 sq mi)|
|- average||36.41 m3/s (1,286 cu ft/s)|
|- left||River Ara|
|- right||Clydagh River|
The Aherlow River rises in Anglesborough, County Limerick, flowing westward towards Ballygibbon and then turning north, meeting two tributaries in Lyre and flowing northwards near Galbally, County Limerick. It then turns eastwards, passes under the R663 and for 5 km forms the County Limerick–County Tipperary border. It enters the Glen of Aherlow then, and follows a meandering course through the Glen, meeting the Clydagh River at Pollagh and another tributary at Ashgrove, where it passes under the five-arch Ashgrove Bridge. The Aherlow River continues eastward, passing under the Limerick–Rosslare railway line in Tankerstown and is bridged by the N24. It meets the River Ara and another tributary at Ballymorris, and flows on southeastwards, flowing into the Suir at Kilmoyler.