The Derry River rises just south of Hacketstown

Hacketstown

Hacketstown , historically known as Ballydrohid , is a town in County Carlow, Ireland.It is located on the R747 regional road at its junction with the R727...

, County Carlow

County Carlow

County Carlow is a county in Ireland. It is part of the South-East Region and is also located in the province of Leinster. It is named after the town of Carlow, which lies on the River Barrow. Carlow County Council is the local authority for the county...

. It flows southeast to Tinahely

Tinahely

Tinahely is a village in the barony of Ballinacor South, County Wicklow in Ireland. It is also a parish in the Roman Catholic Diocese of Ferns.- Location and access :...

, being accompanied by the R747

R747 road

The R747 road is a regional road in Ireland running north-west/south-east from the M9 near Ballitore in County Kildare to Arklow in County Wicklow, a distance of .File:IMG_R747westward5075w.jpg|thumb|350px|left|The R747 road...

 regional road

Regional road

A regional road in Ireland is a class of road not forming a major route , but nevertheless forming a link in the national route network. There are over 11,600 kilometres of regional roads. Regional roads are numbered with three digit route numbers, prefixed by "R" A regional road in Ireland is a...

 for the distance.

South of Tinahely it turns sharply and flows southwest through Shillelagh

Shillelagh

The word shillelagh can mean:* Shillelagh , a type of hand weapon* Shillelagh, County Wicklow, a town in Ireland* The MGM-51 Shillelagh missile* Shillelagh, the code name for Source Mage GNU/Linux 0.5* It's the Same Old Shillelagh, an Irish song...

, briefly forming the border between County Wicklow and County Wexford

County Wexford

County Wexford is a county in Ireland. It is part of the South-East Region and is also located in the province of Leinster. It is named after the town of Wexford. In pre-Norman times it was part of the Kingdom of Uí Cheinnselaig, whose capital was at Ferns. Wexford County Council is the local...

, before becoming the border between County Wexford and County Carlow

County Carlow

County Carlow is a county in Ireland. It is part of the South-East Region and is also located in the province of Leinster. It is named after the town of Carlow, which lies on the River Barrow. Carlow County Council is the local authority for the county...

.

It flows under Clonegal Bridge at a point where it divides Clonegal

Clonegal

Clonegal, officially Clonegall , is a village in the southeast of County Carlow, Ireland. It is in a rural setting, 5 km from Bunclody, County Wexford, 22 km from Carlow and 17 km from the proposed interchange of the N9 and N80 roads at Rathcrogue. It is just over a mile north of...

, County Carlow to the west from Watch House Village

Watch House Village

Watch House Village is a small village in County Wexford, Ireland, on the River Derry. It is a twin village of the much larger Clonegal in County Carlow on the other side of the river, which forms the county boundary.-History:...

, County Wexford, to the east.

A few kilometers further downstream it flows into the River Slaney

River Slaney

The Slaney is a river in the southeast of Ireland. It rises on Lugnaquilla Mountain in the western Wicklow Mountains and flows west and then south through counties Wicklow, Carlow and Wexford, before entering St George's Channel in the Irish Sea at Wexford town...

.

Derry Water River is a separate tributary, which rises near Tinahely and flows north-eastwards to form the Aughrim River

Aughrim, County Wicklow

Aughrim is a small town in County Wicklow, Ireland. It lies in a scenic valley in the east of Ireland where the Ow and Derry rivers meet to form the Aughrim river...

.