[CholeraJoe]

And were they named Williams?

Or Diones(Jones), Mwr(Moore) or Powys (Powers)? My grandparents carried these names to the US from Cumbria (Cymru)

[Myrddin]

Not to mention:
ab Owain -> Bowen
ab Eynon -> Beynon
ap Rhys -> Price
Rhys -> Reese
Gruffudd -> Griffith
Huws -> Hughes
Rhisiart -> Richard
ap Hywel -> Powell
ap Henri -> Penri

Many Welsh surnames are genitive forms of given names. The occurred when the English forced the Welsh to take surnames instead of the old patronymic pattern. Thus we have Johns, Edwards, Williams, Hughes

[Ciexyz]

Did they look like Ioan Gruffudd?