Rhiannon reached more people through a four minute rock song than through eight centuries of manuscript transmission. Stevie Nicks wrote Rhiannon, recorded by Fleetwood Mac and released on the band's self titled 1975 album, after reading Mary Leader's 1972 novel Triad, in which a character named Rhiannon is possessed by the spirit of an old Welsh witch, a plot that gave Nicks the name and the atmosphere of the song well before she had read the Mabinogi or learned of the medieval figure behind it. Nicks has said in interviews that she only later discovered the deeper Welsh material and came to feel a strong connection to it, incorporating spoken introductions about Welsh mythology into live performances of the song in subsequent decades, effectively working backward from a modern fictional echo to the medieval source rather than adapting the Mabinogi directly.
The song became one of Fleetwood Mac's signature numbers and introduced the name Rhiannon, and some version of its mythic association with horses, the otherworld, and a compelling, otherworldly woman, to a global audience with no prior exposure to Welsh literature, and it coincided with, and likely contributed to, a marked rise in Rhiannon as a given name in English speaking countries from the later 1970s onward. The medieval Rhiannon of the First Branch, patient under an unjust penance and ultimately vindicated, and the swirling, enigmatic Rhiannon of Stevie Nicks's lyric share little beyond a name and a horse, but the song remains, for many people, the reason they have heard of her at all.