From sources in the Otherworld (the realm of the deities and possibly also of the dead.), water flows into our world to fill springs or gush forth as rivers such as the Boyne and Shannon. The well itself was viewed as a shrine dedicated to the miraculous emergence of living water and these bodies of water are closely identified with goddesses - Brigid, Bóann and Sionann, for example, who are thought to be part of the water's flow. Such sacred water sources are also often linked to the fruit of certain trees, such as the hazelnut.
It was believed that drinking from these holy waters or bathing in them would bestow the power of the Otherworld in the form of poetic inspiration, wisdom, or healing. Supernatural fish, especially salmon or trout, are still said to appear in a well's depths to those seeking omens for the future. The fish motif may derive from a belief that well goddesses could take the form of a fish. Also, salmon were often credited with being bearers of the insight and wisdom that comes from a supernatural encounter, rather than the knowledge acquired through conventional study.
The holy wells were, in fact, such popular places of worship in pagan times, that the early Roman Church took great pains to stamp them out. But, as is the way with an insuppressible archetypal force, the form changed while the essential mystery continued unaltered: the well as pagan temple metamorphosed into a Christian shrine.