Lough Derravaragh (Irish: Loch Dairbhreach) is a lake in County Westmeath, Ireland, north of Mullingar between Castlepollard, Crookedwood and Multyfarnham.

Turgesius the Viking, renowned for taking Dublin, also possessed strongholds nearby, southwest of Lough Lene.

Knockeyon, the hill of Saint Eyon, is located on the south-eastern shore of the lough, rising steeply to 215 metres.

At this location, far away from the eyes of the world, Cauragh desired to pass the remainder of his life praying to God and fasting.

When the pilgrimage concluded, everyone present adjourned to a green area at the foot of the hill where musicians played and everyone spent the remainder of the day picnicking, drinking and dancing.

The Inny, which is a major tributary of the River Shannon, flows into and from Lough Derravaragh on its northwest shore.

On the cutover on the southeast shore of the lake there are areas of downy birchy (Betula pubescens) woodland, with patches of gorse (Ulex europaeus).

Fringing the lake's shores, a range of habitats has been created as a result of drainage of the River Inny.

The lake shore is a mineral-rich substrate and several plant species of poor fen habitats occur in abundance, such as black bog rush Schoenus nigricans and long-stalked yellow-sedge Carlex lepidocarpa.

[2] Three species (Greenland white-fronted goose (Anser albifrons flavirostris), whooper swan (Cygnus cygnus), and golden plover (Pluvialis apricaria)) are listed on Annex I of the directive,[5] requiring special conservation measures concerning their habitat in order to ensure their survival and reproduction.