A small fault-defined block of Ballyshannon Limestone occurs in Kiltierney Deer Park, just west of Parkhill Lough, about 4 km north of Irvinestown. The Ballyshannon Limestone is so impure and seamed with silty mudstones that karst features are rare, caves in particular being unknown except at this locality.
The stream from Parkhill Lough sinks into a pool which appears to drain underground. A dry valley trending south west has a spring in its northern bank which, after a short surface flow, drains below ground. The water of this system resurges 300 m to the south west in an L shaped karst depression. It then flows across its floor for about 10 m before entering Fiddler's Cave. In periods of heavy rain the sink is overwhelmed and water flows directly down the dry valley to Fiddler's Cave.
The entrance passage of the cave is a stream cleft about 10 m long giving access to a bedding plane about a metre high by 6 m wide and 30 m long, usually filled to within centimetres of the roof. At its downstream limit the water closes with the roof creating an impassable sump. The floor of this section is covered by mud and gravel. The stream resurges 350 m to the south 25 m beyond a pool.
There is evidence of water flow on the surface in the early development of the system. An old dry river bed meanders south, then south west, to the lowest resurgence and a second dry river bed curves in a westerly arc towards the same point.
This is the only known cave in the Ballyshannon Limestone and is of sufficient interest to merit a measure of protection.