There was a medieval English form of this name, Helewis, though it died out after the 13th century. In the 19th century it was revived in the English-speaking world in the form Eloise.
According to tales first recorded in Old French in the 12th century, Yseut or Ysolt was an Irish princess betrothed to King Mark of Cornwall. After accidentally drinking a love potion, she became the lover of his nephew Tristan. Their tragic story, which was set in the Arthurian world, was popular during the Middle Ages and the name became relatively common in England at that time. It was rare by the 19th century, though some interest was generated by Richard Wagner's opera Tristan und Isolde (1865).
Though it has never been popular in the English-speaking world among Christians, it has historically been a common name for Jews, who have used it as an Americanized form of names such as Isaac, Israel and Isaiah.
As an English name, it has been used occasionally since the 12th century. It is the name of a central character in Shakespeare's play The Two Gentlemen of Verona (1594).