Broad Lough is the name given to a brackish lake which lies approximately 1 km north of Parnell Bridge in Wicklow Town. It is included within the boundaries of the The Murrough Wetlands Special Area of Conservation and Special Protection Area (for birds) because of the wetland habitats and species it supports. Broad Lough is generally classified as a transitional water and was created by a shingle ridge, which divides the River Vartry estuary from the open sea. Broad Lough is tidal and extremely shallow. The Lough is surrounded by what is known as a coastal salt marsh. The Lough itself and the surrounding marsh both support an enormous diversity of life, both aquatic and terrestrial. There are a number of important and protected bird species, both resident and transient in the Murrough Wetlands, and the Lough itself is an important breeding ground or juvenile refuge for a number of fish species, notably thick lipped grey mullet. In 2008 inland fisheries Ireland recorded juvenile Gilthead bream in Broad Lough, a species more common to the Mediterranean.
According to The Life of St. Patrick and His Place in History the place where St. Patrick landed was at Inverdea, the mouth of the River Vartry in County Wicklow.
Besides the Vartry River, several smaller streams feed directly into Broad Lough: