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. ==Wildlife==