Marshal, Isabella (p. 1171–1220), was daughter of Richard de Clare (qv) (Strongbow), earl of Strigoil and lord of Leinster, and his wife Aífe (qv), daughter of Diarmait Mac Murchada (qv), king of Leinster. She became the sole heir of the lordship of Leinster on the death of her brother Gilbert (1185 × 1189). Although her mother, ‘the Irish countess’, was still alive, Isabella was placed in royal custody and her lands taken into the king's hands. Shortly before his death, Henry II (qv) arranged the marriage of Isabel to William Marshal I (qv), although it was actually Richard I who allowed it to proceed. She married Marshal in London, where she had been living in the Tower under the supervision of Ranulf de Glanville. Marshal was recognised as lord of Leinster in right of his wife (1189), and this led to controversy with John (qv), lord of Ireland, over grants made during the minority.
Between 1190 and 1210 Isabella gave birth to five sons, each of whom became earl of Pembroke in succession, and five daughters, among whom the Marshal lands were divided in 1247. William Marshal was recognised as earl of Pembroke by King John in 1199, a move that recognised the claim of Isabella's family to the earldom. She travelled regularly with her husband and appears to have become one of his foremost advisers. Her status as the daughter of Strongbow was vital to any hopes that Marshal had of governing Leinster in the first decade of the thirteenth century. She remained in Leinster in the winter of 1207–8 when Marshal returned to England to settle with the king, and emerged as an important figure in her own right during her husband's absence as she directed the defence of Kilkenny during a siege by the justiciar, Meiler fitz Henry (qv). In February 1208 fitz Henry was defeated near Thurles and surrendered himself to the countess, who demanded hostages for his good behaviour. Isabella was an active patron of clerical foundations and approved a grant of land that allowed the foundation of Duiske.
Her husband died in 1219 and was buried in the church of the New Temple in London; his will provided that the lands he held in right of his wife should revert to her for her lifetime, leaving his eldest son only with the patrimonial and acquired lands. Isabella died in 1220 and was buried in Tintern Abbey, Monmouthshire.