Sprawling over 240 hectares, Killykeen Forest Park, 12km northwest of Cavan, has various nature trails (from 1.5km to 5.8km) that lead you through the woods and along the shore of Lough Oughter, which splatters across the map like spilt steely grey ink. It's popular with anglers. Keep an eye out for stoats, badgers, foxes, grey squirrels and hedgehogs, as well as some amazing birdlife.
Many of the low overgrown islands in the lake were crannógs (fortified, artificial islands). The most spectacular is home to Clough Oughter Castle, a 13th-century circular tower perched on a tiny speck of land. It was used as a lonely prison, then as a stronghold by rebel leader Owen Roe O'Neill before being destroyed by Cromwell's army in 1653. The castle lies out of reach over the water; canoe trips run by the Cavan Canoe Centre head here. The forest trails provide wonderful views.