Perhaps I should really be trying the Cornwall mailing list with this
one but here goes.
Dafydd ap Madog o'r Hendwr (died 1351), who held lands in Anglesey and
Caernarfonshire was married to a Cornish lady called Sibylla, the
daughter of a John Cornwall of Brannel, near Launceston. It is this
Sibylla's ancestry that I would be interested to know more about. I know
that there was a family called Cornwall, living at Brannel, which was
descended from Richard, Earl of Cornwall, a son of King John, so I
wonder if Sibylla is part of this same family.
I gather that Dafydd's nephew, Hywel ap Goronwy of Penmynydd, was a
ringleader in the assassination of Henry de Shalford, and was arrested
in Chester, and then imprisoned in Launceston Castle, which belonged to
the Prince of Wales as Duke of Cornwall, and was far from Gwynedd.
Dafydd ap Madog was one of Hywel's sureties, and may have met his
Cornish wife through a visit to Cornwall in connection with his nephew's
imprisonment.