The river's name derives from the mythological figure Ethniu or Eithne, who reputedly died in rapids in its lower reaches.

[1] The Inny begins as a stream at Slieve na Calliagh, in the townland of Tubride, near Oldcastle,[2] a hilly area which holds the complex of megalithic tombs now more often named for Loughcrew, all in County Meath.

It flows under the bridge at the village of Finea, and into Lough Kinale, where counties Cavan and Westmeath meet Longford.

At nearby Tenelick the mythological Princess Eithne drowned in the rapids, giving her name to the river,[1] which powered two mills here for many years.

[2][8] In the 1960s the river was drained which removed the weir at Newcastle, this fed a generator and a sawmill for the "big house" nearby.