Lough Mahon is a large expanse within Upper Cork Harbour, stretching from Blackrock to Passage West and incorporating the estuary of the Douglas River. Several of the city's Cork City’s southern suburbs, including Blackrock, Mahon, Douglas and Rochestown lie along its shores. Little Island, one of the major centres of chemical and pharmaceutical activities in the country, is on the eastern side of Lough Mahon.

Lough Mahon is considered to be the second most valuable bird habitat in the harbour. At low tide, its extensive mudflats are widely used as feeding grounds for waders. Damp grassland on the southern side extends to some low islands which are submerged in very high tides. The area also serves as a high tide roost for waterfowl. Four species are considered to occur in nationally important numbers in Lough Mahon: shelduck, red-breasted merganser, golden plover and black-tailed godwit.

A very attractive walk has been developed along the old railway line from Blackrock to Harty’s Quay, Rochestown and from Hop Island, Rochestown to Passage West.

The Jack Lynch Tunnel crosses under the River Lee from Mahon to Dunkettle. Completed in 1999, it transports vehicular traffic in uni-directional bi-lanes. In 2005, it was estimated that some 40,000 vehicles were using the tunnel daily. Although footpaths are provided, they are for emergency use only and cyclists and pedestrians are forbidden to use the Jack Lynch Tunnel.