It rises on Lugnaquilla Mountain in the western Wicklow Mountains and flows west and then south through counties Wicklow, Carlow and Wexford for 117.5 km (73 mi),[4] before entering St George's Channel in the Irish Sea at Wexford town.

[5] The long-term average flow rate of the River Slaney is 37.4m3/s[5] Towns that the Slaney runs through include Stratford-on-Slaney, Baltinglass, Tullow, Bunclody, Enniscorthy and Wexford.

In Wicklow, herds of deer can be seen, as well as swans, dippers, mallards, herons and kingfishers.

At dusk, bats, owls and otters may be seen, while the mudflats of the estuary are favoured by black-headed gulls, redshanks and oystercatchers.

[8] Ptolemy's Geography (2nd century AD) described a river called Μοδοννος (Modonnos, "mudflats") which may have referred to the River Slaney, though scholarly opinion remains divided on the issue.