Fabulous well or spring, thought to be the common source of both the Boyne and the Shannon rivers, and a source also for supernatural knowledge. Like Connla's Well, which it resembles and may be identical with, the Well of Segais is surrounded by nine hazel trees, whose nuts fall in the water, feeding salmon. When Boand defies the magical powers of Segais, its waters rise up in anger, mutilating her, turning her into the River Boyne. At Connla's Well a comparable story is told of Sinann and the Shannon. Although the Boyne and the Shannon do not have a common source, the name may come from a district adjoining the Boyne that is an affluent of the Shannon. Folk motif: D1811.1.2.
See Vernam Hull, ‘Early Irish Segais’, Zeitschrift für celtische Philologie, 29, H. 3/4 (1964/5), 321–4.