Hello Wendy,
Census records are available for the IOM from Brian Lawson for years 1841, 1851, 1871 and 1891. They are sorted alphbetically and by age. These records were my source. Check his website:
www.lawsons.ca

Also the IOM Family History Society publishes Census records by parish. 1851 and 1881 are available for Marown. 1851 might be worth a peek as it may give occupation and the relationship between people living at the same residence. The FHS also publishes Parish Burial Records and Monumental inscriptions. Just look up publications on the Family History Page.
I would suggest that you consider asking someone to look up Dinah Cottier the original Marown Parish christening records for years 1810-1813, in cage the IGI has an omission. In fact it would be worth looking for all Dinahs, as she could have become Cottier by marriage.
Having composed all the above, which I hope will be useful for other searches, I tried one final trick.
On the IGI, I entered:

Last name: Cottier
Event: All
Year range: + or - 2 years
Year 1812
Region: British Isles
Batch number: C038221 (Christening batch for Marown Parish)
leave all fields blank and voila: DIANA COTTIER
christened October 30 1812.
Why didn't I think of that before!
Bruce