You could send for Mary Ann HILLMAN's birth certificate (hoping this could be the correct parents), this would give the mother's maiden name.

Quarter: January/February/March 1841

Surname: HILLMAN

First Names: Mary Ann

District: Ross

Volume: 26

Page: 199

or even send for Henry HILLMAN's birth certificate J/F/M Qtr 1845 Ross District
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Couple of Public Members Trees on Ancestry for this family : no-one has given Mary's maiden name.