Person:Margaret Deincourt (1)

Name^[1]	Margaret Deincourt
Gender	Female
Birth^[1]	Est 1275	Blankney, Lincolnshire, England
Marriage	Bef 1303	to Robert Willoughby
Death^[1]^[2]	Bef 18 Oct 1333	Willoughby, Lincolnshire, Englandshortly before
Reference Number		Q18756954 (Wikidata)

References

↑ ^1.0 ^1.1 ^1.2 Allen and Susan Mallory.
↑ Some Corrections and Additions to the Complete Peerage: Vol 12, Part 2: Willoughby de Eresby (Proposed Corrections).

http://www.medievalgenealogy.org.uk/cp/p_willoughbydeeresby.shtml#p658

Volume 12, part 2, page 658:
He [Robert de Willoughby (d. 1317)] m., in or before 1303, Margaret, da. of Sir Edmund (DEINCOURT), 1st LORD DEINCOURT, by Isabel, da. of Sir Reynold DE MOHUN, of Dunster, Somerset.

According to Complete Peerage, vol.4, p.120, Isabel, the wife of Sir Edmund Deincourt, was the daughter of Sir Reynold Mohun by his wife Isabel, daughter of William (de Ferrers), earl of Derby. But the issue of Isabel de Ferrers had certainly failed by April 1327, when an inquisition was taken after the death of John Meriet, who had married her granddaughter. At this time Margaret was still living. Clearly, either she was the daughter of another wife of Edmund Deincourt, or Edmund's wife Isabel was not the daughter of Isabel de Ferrers.
Cokayne, George Edward, and Vicary Gibbs; et al. The complete peerage of England, Scotland, Ireland, Great Britain and the United Kingdom, extant, extinct, or dormant [2nd ed.]. (London: St. Catherine Press, 1910-59)
12:2:657-8.

He [Robert de Willoughby] m., in or before 1303, Margaret, da. of Edmund (Deincourt), 1st Lord Deincourt, by Isabel, da. of Sir Reynold de Mohun, of Dunster, Somerset. He d. shortly before 25 Mar. 1317. She d. shortly before 18 Oct. 1333.