Llyr half-speech (or "half-national" or "lisping Llyr"*) is almost certainly a mythological ancestor - he was the father of Bran, Branwen and Manawydan (Manannán Mac Lir) amongst others, all of who appear in early Welsh stories.

He himself has a variety of ancestries - Bromwich* gives two:

Llŷr son of Ffaraon son of Dd.(Dandde) son of Mael Dicko son of Kryf son of Dyfnwal Moelmud
Llŷr son of Paran son of Kerihir Llyngwyn son of Keidio Bywrmwyn son of Arthan son of Meirion son of Gerient son of Graydiol son of Dyngad son of Albean Tirvad son of Anny son(?) of Galbean

The text associated with the latter places him at 55 BCE.

Bromwich* rapidly dispatches the notion, held later, that Llŷr was the original King Lear. However, she takes the view there may be evidence for a link between Llŷr and Welsh hero Llud Llaw Ereint: Llŷr appears as one of the three "exulted prisoners of the Island of Britain" in the triads in the Red Book of Hergest (see Other Works), but the same triad in "Culhwch and Olwen" (Details) has Llud in his place. Llud, in turn, is likely to be based on No(/u)do(/e)ns , a British God worshiped at the Romano-British temple at Lydney Park, Gloucestershire (Info). The connection suggested for all three is via an Irish original Nuadu and the fact that both Nodons and Llŷr appear associated with the sea.

Llud was son of Beli Mawr (the celtic god Belenos), the most over-used of all ancestors, who was, in turn, son of Manogan or possibly Dyfnwal Moelmud (Britain's legendary law-maker), with his mother (or wife) being Anna. Anna, just to take the all-inclusiveness of this to its naturally absurd ends, was the cousin of the virgin Mary, though might also be Danu - the Irish mother of the gods*.

Either way, Arthur's family had quite a start in life.