This seems to be a
German pet form of
Elizabeth. It is listed as such at the following website about names in Hamburg, Germany from the 1400s:
http://www.ellipsis.cx/~liana/names/german/hamburg.html
Ilsabe is also listed as a form of
Elisabeth in
Knaurs Vornamen Buch by
Margit Eberhard-Wabnitz &
Horst Leisering.