Crehan lies within Upper Lough Erne in County Fermanagh.  The Upper Lough is a large and complex freshwater system formed after the last ice age. As the glaciated landscape flooded, the drumlins formed a mosaic of islands and shallow lakes bordered by damp pastures, fens, reedswamp, alder/willow carr and oak woodland.  There are estimated to be around 90 lakes in Upper Lough Erne.

Upper Lough Erne is recognised as a European Special Protected Area (SPA)1. The SPA is host to internationally important numbers of wintering whooper swans, and Greenland white-fronted geese, as well as nationally important wintering wildfowl species including the cormorant, mute swan, tufted duck, wigeon, teal, goldeneye, coot, mallard.  It also supports an important collection of breeding birds including common tern, great crested grebe, curlew, snipe and redshank.

Specifically, Crehan is designated as part of the Dernish Island Area of Special Scientific Interest (ASSI)2, awarded because of its precious wetland flora and fauna.  The vegetation on and around the island is very diverse, with well-developed areas of swamp and fen. Stands of common reed and bulrush at the water’s edge give way to a fen zone which is particularly rich in sedge species. Lesser tussock sedge, bottle sedge and bladder sedge are dominant. Other species occurring include water parsnip, flowering rush, creeping Jenny and marsh stitchwort.

The grassland behind the fen zone is also rich in plant species such as quaking grass and a number of smaller sedge species such as carnation sedge, glaucous sedge and flea sedge. The island also supports a high density of breeding waders. Species include lapwing, curlew, redshank and snipe. Other breeding birds of note include great crested grebe, mallard and shoveler.