Castlerahan (Irish: Caisleán Raithin[1]) is a barony in County Cavan, Ireland.

The name is derived from Irish Caisleán Raithín, "stone fort of the little ringfort," although other writers link it with raithean, "bracken", or with a Norse Gael leader named Raithin.

[5][6] Castlerahan is located in the southeast of County Cavan, the area surrounding Lough Ramor.

It contains the parish of Munterconnaught, named for Cu Connaght Ua Raghallaigh (O'Reilly).

[8] The barony of Castlerahan was created by 1609 in the Plantation of Ulster, and was archaically spelled Castlerachan.