Margery Byset (Bisset, Bissett; also Marjery, Margaret, Marie) was an Irish noblewoman belonging to the Bissett family whose marriage to John Mór Tanister MacDonnell in 1399 laid the basis for the Clan Donald claim to the Glens of Antrim, the lordship of which her family had established in the 13th century.
[1] This is all that is relatively certain, however, because no medieval Bissett pedigree has survived, the family falling from power in the Glens of Antrim in or not long after 1522, following the Battle of Knockavoe, and not being recorded by Duald Mac Firbis in the mid-17th century except in reference to their maternal kin the MacDonnells, who replaced them.
Mac Firbis uniquely describes the Bissetts as being of Greek origin, first arriving in England with William the Conqueror.
[2] As far as Margery's likely mother Sabia it is possible she was one of the four daughters of Aodh Reamhar Ó Néill, King of Ulster, whose names were unknown to the 19th century genealogist John O'Hart,[3] the O'Neill pedigrees themselves being imperfectly preserved.
He was naturally dismissed as an impostor by Henry IV, but had a kind of career as an anti-Lancastrian figurehead anyway and died in 1419.