Some corrections and additions to the Complete Peerage: Volume 10:
Volume 10, page 121 (as modified by volume 14):
He [James (le Botiller), Earl of Ormond (d. 1382)] m. Elizabeth, da. of his guardian, Sir John DARCY [LORD DARCY], of Knaith, presumably by his 2nd wife, Joan, widow of Thomas FitzJohn (FITZGERALD), EARL OF KILDARE, and 4th da. of Richard (DE BURGH), EARL OF ULSTER [I.].
The identity of Elizabeth's mother is confirmed by the statement that her grandson, James Butler, earl of Ormond (d. 1452), and his wife Elizabeth FitzGerald were related on the side of their fathers in the 3rd degree of kindred [Douglas Richardson, June 2002, citIng Calendar of Papal Letters, vol. 8, pp. 442, 443]. It seems that the only possibly route for this relationship is through a common descent from Joan de Burgh, who was ancestress of the FitzGeralds through her previous marriage mentioned above.