Shannon Cave is an active stream passage cave situated in County Fermanagh, Northern Ireland and County Cavan in the Republic of Ireland. At 130 metres (430 ft) vertical depth, it stands joint 6th with Poulnagree in County Clare in the deepest caves of Ireland. The cave is recorded at 5.4 kilometres (3.4 mi) in length (making it 6th longest on the island of Ireland), but exploration is ongoing and further passage is expected to be found.
The cave consists in the main of a large mainstream passage running to a terminal sump. The water flowing in the cave has been dye traced to Shannon Pot, two kilometres to the west of the terminal sump, which is the traditional source of the River Shannon. Inlets to the main passage are numerous. Some, such as the large "Mistake Passage" have been pushed only a short distance. Since the cave is undergoing active exploration, there remain many issues to be sorted out and the sinks for many of these inlets remains unclear.
The stream passage itself is of considerable proportions in parts, but also contains many higher level fossil passages which have been deserted by the stream. The cave contains numerous calcite formations, with large amounts of helictites in places. Boulder chokes are frequent with several parts of the cave being quite unstable, especially the JCP passage.
While the location of the entrance to the cave is in County Fermanagh, the majority of the cave lies under County Cavan. The original entrance to the cave was in Cavan, however this is now inaccessible. The current entrance of Polltullyard is located north of the Marlbank area of Fermanagh, high in the moors.