Isolda, daughter and heir of Sir Thomas Mountjoy, son and heir of Sir Ralph Mountjoy. [Burke's Peerage]
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He [John le Blount] married Isoude (i). He died in 1358. [Complete Peerage IX:331, (transcribed by Dave Utzinger)]
(i) According to tradition, Isoude de Mountjoy. The mystery of her parentage has not been solved. The release by John le Blount in 1374 to his brother Walter of all his rights in the lands in Gayton, Yeldersley, Brushfield, etc., where were Mountjoy manors in the 13th and 14th centuries, suggests that a portion of the Mountjoy estates had descended by inheritance to John, then eldest surviving son of John le Blount and Isoude. It was a portion only, because (i) Gayton and Yeldersley, etc. descended, throught the marriage of Isoude (daughter and heir of Serle de Mountjoy, son and heir of Ralph de Mountjoy) to Robert de Ireland; the family of Ireland were still holding temp. Henry VII; (ii) the receipt given by Madam Wake in 1359 for evidences belonging to Richard Blount, the young heir of John and Isoude, refers to vint oyt feetes en un boist del heritage la mere le dit Richard et ses parceners des tenements en le Pek, etc.