Shannon Pot, Source lake of River Shannon in County Cavan, Ireland

Shannon Pot is a circular spring pool at the base of a limestone cliff and marks the source of Ireland's longest river. The water emerges from natural openings in the rock and collects in a clear, deep pool surrounded by steep banks.

The earliest written record of Shannon Pot is stanza 2 of poem X in the Book of Magauran, composed around 1349 by Giolla na Naomh Ó hUiginn, which calls it the well from which the Shannon flows. In 1971 divers Roger Solari and John Elliot first explored the pool to 6 m and then 9 m. In 2008–2009 Artur Kozłowski, Paul Doig and Alasdair Kennedy surveyed the chamber to 14.6 m.

The pool's Irish name, Log na Sionna, means 'hollow of the Shannon'. Legend says the river is named after Sionnan, granddaughter of the sea god Manannán mac Lir, who drowned here while eating the forbidden fruit of the Tree of Knowledge planted by the druids; the welling water then flowed over the land as the river.

The site is reachable by marked walking paths from a parking area, with information boards explaining the natural features. Visit in dry weather as the paths around the pool can become slippery due to moisture and moss.

In high water, dye tracing has shown the pool is linked underground to Badger Pot and Pigeon Pot, 10.6 km north near Florence Court Forest Park in County Fermanagh. Its permanent catchment covers 12.8 km² of the Cuilcagh slopes, including Garvagh Lough, whose water sinks at Pollnaowen before rising here.