http://www.patpnyc.com/hrolf-de.shtml#b
http://www.pwestern.f9.co.uk/html/dat29.html#0
http://www.castlewales.com/mar_chld.html:
Isabel married Gilbert de Clare, seventh earl of Clare and fifth earl of Hertford, on October 9, 1217. This was soon after her father had captured Gilbert at the battle of Lincoln in May 1217. Gilbert was thirty-seven years old, but Isabel’s age in not known. Between 1217 and 1230, when Gilbert died, they had six children. These children were: Richard (d1262) who married Margaret, daughter of Hubert de Burgh, and secondly Maud, daughter of John de Lacy; William (dsp 1258); Gilbert (dsp?); Amicia (d1283) who married Baldwin de Redvers; Agnes (dsp 1226); and Isabel (d?) who married Robert de Brus. In April 1231, William the younger married his sister Isabel, widow of Gilbert de Clare, to Richard earl of Cornwall and brother to King Henry III. Isabel died in October 1239, and her grandson Gilbert "The Red" inherited her lands as well as the de Clare lands on the death of his father, Richard, in 1262.