Here is a detailed examination on What Does the Name Isabelle Mean.
ISIBÉAL, genitive idem (the same), Isabella, Sybil, Sibby, Elizabeth, Eliza, Bessie, (Annabel, Annabella, Bella); the French form of Elizabeth (see Ailís); apparently the form in which the name first came into Ireland; still in use, but rare; also Sibéal.