Llŷr in the context of "Brân the Blessed"

Celtic mythology is the body of myths belonging to the Celtic peoples. Like other Iron Age Europeans, Celtic peoples followed a polytheistic religion, having many gods and goddesses. The mythologies of continental Celtic peoples, such as the Gauls and Celtiberians, did not survive their conquest by the Roman Empire, the loss of their Celtic languages and their subsequent conversion to Christianity. Only remnants are found in Greco-Roman sources and archaeology. Most surviving Celtic mythology belongs to the Insular Celtic peoples (the Gaels of Ireland and Scotland; the Celtic Britons of western Britain and Brittany). They preserved some of their myths in oral lore, which were eventually written down by Christian scribes in the Middle Ages. Irish mythology has the largest written body of myths, followed by Welsh mythology.

The supernatural race called the Tuatha Dé Danann is believed to be based on the main Celtic gods of Ireland, while many Welsh characters belong either to the Plant Dôn ("Children of Dôn") or the Plant Llŷr ("Children of Llŷr"). Some figures in Insular Celtic myth have ancient continental parallels: Irish Lugh and Welsh Lleu are cognate with Lugus, Goibniu and Gofannon with Gobannos, Macán and Mabon with Maponos, and so on. One common figure is the sovereignty goddess, who represents the land and bestows sovereignty on a king by marrying him. The Otherworld is also a common motif, a parallel realm of the supernatural races, which is visited by some mythical heroes. Celtic myth influenced later Arthurian legend.

Lir or Ler (meaning "Sea" in Old Irish; Ler and Lir are the nominative and genitive forms, respectively) is a sea god in Irish mythology. His name suggests that he is a personification of the sea, rather than a distinct deity. He is named Allód in early genealogies, and corresponds to the Llŷr of Welsh mythology. Lir is chiefly an ancestor figure, and is the father of the god Manannán mac Lir, who appears frequently in medieval Irish literature. Lir appears as the eponymous king in the tale The Children of Lir.

Branwen, Daughter of Llŷr is a major character in the medieval Welsh story known as the Second Branch of the Mabinogi, which is sometimes called the "Mabinogi of Branwen" after her.

Branwen is a daughter of Llŷr and Penarddun. She is married to Matholwch, King of Ireland, but the marriage does not bring peace.

Manawydan fab Llŷr is a figure of Welsh mythology, the son of Llŷr and the brother of Brân the Blessed and Brânwen. The first element in his name is cognate with the stem of the name of the Irish sea god Manannán mac Lir, and likely originated from the same Celtic deity as Manannán. Unlike Manannán, however, no surviving material connects him with the sea in any way except for his patronymic (llŷr is an old Welsh word for sea). Manawydan's most important appearances occur in the Second and Third Branches of the Mabinogi (the latter of which is named for him), but he is also referenced frequently in medieval poetry and the Welsh Triads.

Penarddun is a figure in Welsh mythology, the wife of Llŷr. The Second Branch of the Mabinogi names Bran, Branwen, and Manawydan as her children by Llŷr, and ascribes to her two additional sons by Euroswydd: Nisien, a good man, and Efnysien, a conniving troublemaker. The Welsh Triads call Llŷr one of the Three Exalted Prisoners of Britain for his captivity at Euroswydd's hands; this likely refers to a lost tradition of the birth of Penarddun's younger sons. The Mabinogi names Penarddun as a daughter of the ancestor Beli Mawr, but the genealogy is confused; it is possible she was meant to be his sister rather than daughter.

The name Penarddun can be translated as "Chief Beauty" or "Most Fair" (Welsh pen "head, chief, foremost" + arddun "fair, beautiful (of a girl)".