On the western side, the Barrow, where it flows by Carlow town, forms for 5 miles the boundary between Carlow and Queen's County; next flows through Carlow for 11 miles; and for 19 miles more forms the boundary between Carlow and Kilkenny.

On the eastern side, the Slaney runs southward through the county for 18 miles, and for 3 miles more forms the boundary between Carlow and Wexford; after which it enters Wexford.

The Burren River rises on the northern slope of Mount Leinster, and flowing northwest, through the middle of the county, joins the Barrow at Carlow.

The Derreen, which enters Carlow from Wicklow, joins the Slaney 3 miles below Tullow: it rises in the southern slope of Keadeen mountain, east of Baltinglass in Wicklow, and is then called the Douglas, flows southwest for some distance, and then forms for a mile the boundary between Wicklow and Carlow, after which it enters Carlow: further on it forms again the boundary between Wicklow and Carlow for five miles, and then finally enters Carlow, ending its course in the Slaney a little further on.

The Clody rises in Mount Leinster, and flowing eastward, joins the Slaney at Newtownburry, running the whole way on the boundary between Carlow and Wexford.

The Mountain River and the Corries River (also called, in the lower part of its course, the Black River or Dinin River) both join the Barrow at Borris.

The Pollmounty forms the extreme southern boundary, and is joined from the northeast by the little river Drummin.

The Lerr rivulet, joining the Barrow 3 miles north of Carlow town, forms a small part of the northern boundary.