Gwern: GOO-irn
Welsh:
Alder
The young son of Branwen and Matholwch, and would have been the future king of Ireland if Efniessin hadn't thrown the boy into a bonfire after the kid refused to hug him. This murder was one of the causes of the great battle which killed all of Ireland--save five pregnant women--and all of the British army, save seven friends.
See The Mabinogion, "Branwen uerch Llyr"