[6] After her brother Gilbert's death, Isabel became one of the wealthiest heiresses in the kingdom, owning besides the titles of Pembroke and Striguil, much land in Wales and Ireland.

[7] Isabel was a ward of King Henry II, who carefully watched over her inheritance, and who in 1189 had confided her to the keeping of Ranulf de Glanville chief justiciar of England.

[9] Marriage to Isabel elevated William Marshal from the status of military captain and knight into one of the richest men in the kingdom and imbued him with aristocratic prestige.

[10] During the first few months of the marriage Isabel accompanied her husband, whilst he served King Richard I, to Normandy, crossing from Dover to Calais on 11 December 1189.

Although Marshal did not become Earl of Pembroke until 1199—a revival of the title by King John as an act of favour—he nevertheless assumed overlordship of Leinster in Ireland and the Marcher lordships of Chepstow and Usk with Isabel's many other estates in several English counties, which belonged to her father's and her own earldom of Striguil.

[10] She may have ruled Leinster in his absence till as late as 1203, with as her seneschal a Wiltshire knight, Geoffrey fitz Robert, who was married to Isabel's aunt, Basilia de Clare, a sister of Strongbow.

[13] She was again left to rule Leinster in 1207-8 during her husband's house arrest at the court of King John when, though pregnant, she successfully led the campaign which defeated the rebel barons of the province.

Isabel lived as a widow for only ten months after the death of William Marshal, though it was by no means an uneventful period, which has left a good deal of evidence as to how a great heiress such as she was, managed her affairs when she came into full control of her inheritance.

[15] There is evidence that she made good use of her eldest son as her agent in managing the great estates that were hers to dispose of in the months she had them, both of them stonewalling her late husband's executors to avoid paying the debts he left.

[18] This identification was subsequently rejected, even before modern research identified her true burial place at Tintern [19] It was suggested in 1892 by Paul Meyer that Isabel might have encouraged the composition of the Song of Dermot, which narrates the exploits of her father and maternal grandfather.

[22] This failure in heirs is supposedly attributed to a curse placed upon William Marshal by the Irish Bishop of Ferns, Albin O'Molloy.