Faelnirv was an elven drink tasting similar to mulled cider mixed with mead. It was made from distilled elderberries and spun moonbeams. It was said that a strong man could travel for three days on naught else: Oromis made some enchanted faelnirv as a gift for Eragon in Eldest before he left to fight in the Battle of The Burning Plains. It gives the drinker, as Orik discovers in Eldest, the gift of loquacion.
Orik, while under it's effects, proclaims it 'the besht and greatest of the elves' tricksty inventions'.
Faelnirv could take on multiple forms. In Inheritance, Arya and Eragon partook in a form of faelnirv with the addition of leaves from a plant that grows along the shores of Rona Lake. This form led to greater intoxication, though this effect could be ended with a spell.
Faelivrin was Gwindor's name for the elf Finduilas in The Children of Hurin (Tolkien).
While unconfirmed by Paolini, the description of the effects of the leaf-augmented faelnirv in Inheritance closely mirror the effects of cannabis for first-time users, including the short-term memory loss, the effects on Eragon's vision (nystigmus) and his reaction to music. "He listened...while fantastical images roiled within his mind, images that the faelnirv had induced but that the music shaped."