Portmore Lough is roughly circular and covers an area of 707 acres. The lough and its shoreland are designated a Ramsar site, a Special Protection Area and an Area of Special Scientific Interest. The lough is now part of a nature reserve of the Royal Society for the Protection of Birds.
The name of the lough is from the Gaelic Loch an Phoirt Mhóir, meaning "Lake of the great landing place". It has an alternative name, too - 'Lough Beg', which means "Little Lake", though in this it may be confused with Lough Beg on the Lower Bann, immediately north of Lough Neagh.
The lough is near the site of the former Portmore Castle, erected in 1664 and removed in 1761. It is also the presumed location of the Portmore Ornament Tree whose demise in a windstorm of 1760 is lamented in the Irish folk song, Bonny Portmore.