The census return which Brian Lawson posted in 2005 looks as though they could have married in England - children born there:

1851, Braddan, district 4, schedule 101, Paper Mill

William Demeza, head, married, 50, paper maker, England
Jane, wife, married, 48, England
Elizabeth, daughter, unmarried, 27, England
John, son, unmarried, 23, paper maker, England
Rachel, daughter, unmarried, 18, England
George, son, unmarried, 16, England
Ann, daughter, 14, scholar, England
Louisa, daughter, 9, scholar, Braddan
William, son, 7, scholar, Douglas
Selina, daughter, 2, Braddan

In the 1841 census George Demenza aged 6 b. England was in Braddan at Bridge House with Edward Gelling 15 & Ann Gelling 35.
I couldn't see the rest of the family.

This Maughold marriage looks like William's son:
1 May 1864
William DEMEZA, Full age, Bachelor, Upholsterer, Ramsey, [father] William Demeza, Papermaker, and
Margaret Ann KILLEY, Under age, Spinster, Ramsey, [father] Robert Killey, Blacksmith. Bride had consent of parents.

This 2005 message says Janes's name was KILLEY:
www.isle-of-man.com/cgi-bin/interests/genealogy/bulletin/index.pl?noframes;read=505051

Sue