The daughter of Earl Richard ‘Strongbow’ Earl of Pembroke and Striguil and Lord of Leinster and Aoife of Leinster, Isabel grew up as part of the powerful de Clare family and following her brother Gilbert’s death became one of the wealthiest heiresses in the kingdom. She was placed by King Henry II, who did not trust Strongbow, in the care of Ranulf de Glanville, justiciar of England. 

In 1189 her marriage was arranged by Richard the Lionheart to William Marshal. The couple were happily married despite a twenty-six year age gap and never having met before their wedding in August that year. Isabel travelled by her husband’s side, took part in the management of their estates and issued writs. They went to Ireland in 1200 and she may have ruled Leinster in his absence. She continued to demonstrate her capacities when, Marshal was placed under arrest in 1207, she led a campaign against the province’s rebel barons.  She was pregnant at the time. She gave William ten children, five boys and five girls. Marshal recognised that his power and his wealth came from his wife, honoured, loved and respected her intelligence. In Leinster her presence in Marshal’s life gave his rule legitimacy – she was after all the grand daughter of the last king of Leinster.

Isabel managed her husband’s affairs in his absence and following his death she took control of her own inheritance corresponding with the justiciar of England, with the papal legate and with King Philip II of France.  After thirty years of marriage, William died. One of the last things he did was to join the Templars – forgoing the company of his wife and daughters in his final days.  Isabel was devastated by Marshal’s death but she worked closely with her family to preserve her inheritance.   She died ten months after her beloved husband. Her earldom did not survive her children. All five of the couples’ sons died without heirs.

Isabel was buried in Tintern Abbey next to her mother.