In a recent post (http://mistshadows.blogspot.com/2017/10/degsastan-and-origin-of-mil-uathmarfer.html), I offered my final identification for Uther Pendragon. It was an old idea, but I'm now convinced it is correct.
However... the conclusion reached in that post is in need of some clarification. For there is some evidence in the Taliesin poetry that while Uther does seem to originate from the mil uathmar/fer uathmar of the Irish "Conception of Mongan" tale, another parallel tradition existed which actually identified him with that story's Manannan son of Lir.
Several scholars (including Bromwich) call attention to the fact that the Cawrnur in the 'Marwnat Vthyr Pen' or "Death-Song of Uther Pen[dragon]" is also mentioned in the Taliesin poem entitled 'Cadair Teyrnon', "The Chair of the Divine Lord." In this latter poem, it would appear Teyrnon (whose name matches that of Teyrnon Twrf Lliant, 'Divine Lord of the Tumultuous/Turbulent Sea', a sobriquet for Manawydan son of Llyr in the MABINOGION tale "Pwyll Prince of Dyfed") is involved in a horse raid on Carwnur and his sons. In this poem, Arthur is mentioned. This has led some (like Thomas Green in his ARTHURIANA: EARLY ARTHURIAN TRADITION AND THE ORIGINS OF THE LEGEND) to wrongly assume that the Teyrnon in question is actually Arthur.
I would make the case instead for Teyrnon in the 'Cadair Teyrnon' being Manawydan (= in this context, Manannan the father of Mongan, who transformed into Fiachra in order to lie with Fiachra's queen).
What we have in Uther, then, is a conflation of two characters from the "Conception of Mongan": the mil uathmar (a character created as an eponym for Degsastan as Egesan stan) and Manannan mac Lir.
Neither were the father of Arthur.