Probably derived from the old Celtic name Rigantona meaning "great queen". It is speculated that this was the name of an otherwise unattested Celtic goddess of fertility and the moon. The name Rhiannon appears later in Welsh legend in the Mabinogion, borne by the wife of Pwyll and the mother of Pryderi.
As an English name, it became popular due to the Fleetwood Mac song 'Rhiannon' (1976).