Though no more than a small to medium bat by European standards, the Leisler's is the largest found in Ireland. It is about three times the weight of the Lesser-horseshoe and its flight dimensions are: wingspan 13in. (33cm); width 4in. (lOcm).

It is a thickly furred, dark brown bat with a conventional looking mammal face, lacking a nose-leaf. The ears are large and broad and contain a club-shaped element or 'tragus'. The size, head shape and particularly the ear structure make identification of this bat straightforward.

Leisler's bats roost in dense groups in the attics of buildings and hibernate in holes in trees or crevices in undisturbed ruins or other buildings. Some colonies are large, such as one of a thousand in west Cork. It is likely that Ireland is their European stronghold.