Strongbow's mother was Isabel (Elizabeth) de Beaumont, sister to Robert earl of Leicester and Waleran count of Meulan. Isabel had been the youngest mistress of King Henry I, and their liaison resulted in a natural daughter, Isabel (Elizabeth), born c.1129/30. When Isabel de Beaumont married Gilbert in 1130, she took this daughter with her. Strongbow was born before the end of 1130; thus he was raised with the natural half-sister of the Empress Matilda.