In Irish mythology, Connla's Well (also called the Well of Coelrind, Well of Nechtan, and Well of Segais) is one of a number of Otherworldly wells that are variously depicted as "The Well of Wisdom", "The Well of Knowledge" and the source of some of the rivers of Ireland. Much like the Well of Nechtan (and some sources conflate the two), the well is the home to the salmon of wisdom, and surrounded with hazel trees, which also signify knowledge and wisdom.
According to the legend of Cormac mac Art, the well stands in the hall of the fairy king Manannán mac Lir. In one legend, the well was home to the one Salmon of Wisdom until it was caught by the poet Finn Eces. Nine sacred hazels of wisdom surround the well, blooming and fruiting in the same hour. Eating the hazelnuts that fell into the well, drinking the well water when the hazelnuts fell in, or eating the salmon that ate the hazelnuts granted knowledge and poetic inspiration. The well was accordingly sought out by many poets and philosophers.
Only the god Nechtan and his cupbearers were meant to approach the well. The goddess Boann approached the well of Nechtan and circled it three times anti-clockwise, causing the waters to rise up and rush to the sea, becoming the River Boyne. Seven rivers of knowledge are said to flow from Connla's Well, including the Boyne, the Shannon, the Nore, the Barrow, and the Slaney.
And when the sun sets dimmed in eve, and purple fills the air,
I think the sacred hazel-tree is dropping berries there,
From starry fruitage, waved aloft where Connla's Well o'erflows;
For sure, the immortal waters run through every wind that blows.
- Meyer, Kuno; Nutt, Alfred (1897). The Voyage of Bran Son of Febal. London: David Nut.