See the second edition of Cokayne's *Complete Peerage*, vol. X, p. 349, note (b).
Isabel became in ward to King Henry II and remained under the royal guardianship for the space of fourteen years, when she was given in marriage to William Marshal, who thereupon became Earl of Pembroke. [Sir Bernard Burke, Dormant and Extinct Peerages, Burke's Peerage, London, 1883, p. 121, Clare, Earls of Pembroke]
Isabel de Clare, only child and heiress of Richard de Clare (surnamed Strongbow), Earl of Pembroke, and justice of Ireland, who had been under the guardianship of Henry II, was given in marriage by King Richard I to William Marshal. [Sir Bernard Burke, Dormant, Abeyant, Forfeited, and Extinct Peerages, Burke's Peerage, Ltd., London, 1883, p. 358, Marshal, Earls of Pembroke]