As I was starting to write a criticism, I received a book by a scholar whose field is ancient Celtic literature. The scholar, Peter B. Ellis, cautions using Lady Charlotte Guest's text. I understand from an academic standpoint. However, Ellis's book targets the popular market, making that, too, a problem. In any case, my criticism will not reach academia; and I read another translation.
I see the events of this tale as occurring immediately after the withdrawal of the Roman troops. From the west came Norse invaders, from the east Irish tribes and marauders. From my (scant) reading, it appears that the area the Irish invaded had been partly depopulated, so that resistance would not be great. In the tale, what is a welcoming of the man and woman with the pot, and the rapid growth of their numbers and fortification of their territory, likely was a remembrance of an invasion put in innocuous terms. The same might have been with Bendigeid's invasion of Ireland. Certainly, in the generation after the withdrawal of Roman troops the Welsh would not have been strong enough to invade Ireland. What might have happened is that the Welsh led a reconquest of their lands, and this was much later misinterpreted as a conquest of Ireland.
The tribes of southern Wales play a subordinate role to those of the north. The southern tribes, notably the Silures, readily adapted to Roman ways. Archeological aerial photography located a dozen Roman villas in the south. The north, occupied by the Ordovices, was garrisoned by the Romans. The Ordovices must have looked askance at the southern Welsh tribes, the territory of Pryderi, and this have found its way into the tale.
Bendigeid Vran (or Bran) is identified with Bron, the Fisher King. Vran is mortally wounded by a poison arrow in the leg. The fisher king is lame. The story of the Fisher King is a most intriguing tale in Medieval literature, but it breaks off before the ending; then, four authors took up the tale. The head of Bendigeid Vran is buried on a hill in London protecting the country from invasion, and later disinterred. This serves as a reason after the fact for why there were invasions. Ellis writes that King Arthur (in legend, of course) disinterred the head, to his regret. Ellis must have his source for this. It is not in the tale.