p. Lionel of Antwerp, 1st Duke of Clarence and Elizabeth de Burgh, 4th Countess of Ulster (d.1363).
She married Edmund Mortimer, 3rd Earl of March (1352-81). 
m. Edmund Mortimer, 3rd Earl of March abt 1368 Reading Abbey
 Death date changed June 25, 2015 from "certain, 1378-01-05" to "before, 1381-02-09". Text has death as "d. 05 Jan 1382 Cork, Ireland" (couldn't tell source). Jan. 1382 and Feb 1381 are not incompatible, if both were originally 1381/1382. However, sources for any/all would be nice.
↑ When Richard resigned crown with no issue on 29 Sep 1399, the rightful heir was Edmund Mortimer, 5th Earl of March, whose father Roger died the previous year. The throne was usurped by Richard and Philippa's first cousin, Henry Bolingbroke, which later precipitated the Wars of the Roses. As a result of her seniority in the line of succession to the throne of the Kingdom of England and her marriage into the powerful Mortimer family, her descendants eventually succeeded to the throne as the House of York under Edward IV.
Richardson, Douglas. Royal Ancestry: A Study In Colonial And Medieval Families, in 5 vols. (Salt Lake City, Utah, 2013): vol. I p. 88; vol. II p. 24; vol. II p. 174.
Richardson, Douglas. Magna Carta Ancestry: A Study In Colonial And Medieval Families, 2nd ed. in 4 vols, (Salt Lake City, Utah, 2011): vol. III p. 193-194.
The American Genealogist [TAG], vol. 69 no. 3 (July 1994): 129-139. The Ancestry of Joan Legard, Grandmother of the Rev. William Skepper/Skipper of Boston, Massachusetts. by James L. Hansen. AmericanAncestors.org (paid) LINK