I've seen the name before but have no idea how it's pronounced.
It's most popular from the song by Fleetwood Mac. Just did a quick search & found this:
Rhiannon Great Queen of the Celts a project by Rowen Saille
© All original material in this site is under copyright protection and is the intellectual property of the author. http://www.orderwhitemoon.org/goddess/Rhiannon.html#top
Origins
Rhiannon [RHEE-awn-on] is a Welsh underworld Goddess form. Her story is told in the Mabinogion ( 1 , translation by Lady Charlotte Guest). Her origin goes back further still according to Jean Markale ( 2 ) who believes that she might have been the original Mother Goddess of the Celtic people. She brings sleep, dreams, and sometimes nightmares (3, 4 ). She is found in several ancient cultures, called by several names (3 ). Through history Rhiannon has survived and influenced many cultures and legends. Her name translates as "divine" or "Great Queen." She may have been sun Goddess (3). She is Goddess of change, movement, and magic (3, 4). She comforts in times of crisis, loss, and illness (5). She gives us gifts of tears, forgetfulness (to promote healing), and humor to ease our sufferings in this life (5) and guides us to the next (3, 6). She is also accompanied by golden birds whose singing can call the dead or grant peaceful sleep to the living (4, 5, 6).
Ritual to Rhiannon
Purpose: To heal grief, loss, or disappointment. To encourage moving forward.
Invoke - "We honor Rhiannon who comforts those in crisis and in times of grief and loss. Her sweet song eases us and gives us the closure we need to move on in our lives."