Ahhh, Robert - and there I was hoping you had her birth certificate in a tome in your attic :-)
The specific birthdate and place: 1555 - Buhy, Val-d'Oise, Île-de-France, France, Gives me hope of some primary source somewhere!
On Jean du Plessis profile, we have this tantalising secondary source:
"Jean du Plessis [Parents] was born in 1548 in France. He died in 1640. He married Judith Anne du Plessis du Mornay about 1575 in France.
Judith Anne du Plessis du Mornay was born about 1555 in Buhv, Vetin, France. She died . She married Jean du Plessis about 1575 in France.
They had the following children:
Charl du Plessis was born in 1600.
Prepared by:Johann Wilhelm Grobler, PO Box 321,Rothdene,1964,South Africa"
On Jean's profile there is also a Family Search record (that I can't replicate with a MH Search) Added by Clara van Rooy Smith - without a primary source that I can see either
Birth: Between Jan 11 1548 and Jan 10 1549 - Poitiers, Vienne, Poitou-Charentes, France
Death: 1640 - Poitiers, Vienne, Poitou-Charentes, France
Son: Charl Prieur DuPlessis
The problem with the MH trees, is they all (?) seem to place Judith as Philippe's daughter.- with the consequent problem of his birthdate being 1549, and hers being c (?) 1555.
So placing her as his sister solves that better,
BUT then we have to account for the fact that the two brothers-in-law Jean du Plessis and Philippe de Mornay, Sieur du Plessis-Marly share a surname/land title - duPlessis.
Not impossible as either a coincidence, or as a land transfer title - but I probably useful to follow the trail of the duPLessis land title (?) - given that Jacques de Mornay, Chevalier, Seigneur de Buhy - the father of Philippe de Mornay, Sieur du Plessis-Marly & Judith Anne de Mornay du Plessis (?maybe) does not seem to have it.
These are things we may have discussed already - I'm just recording my thoughts for possible avenues of research as they come to me.