Registration didn't start until 1849 for marriages (1878 for births and deaths). In the parish registers prior to 1849 you will generally only find the names of the parties, their status (bachelor, widow etc) and whether they were "of" the parish in which the marriage took place or from another parish. Submitting a request through the Manx National Heritage Museum would only get you a certified copy of the parish entry so won't be of much use to you.
Your best bet would be to submit a request for a census lookup for the 1841 census. Brian Lawson will do these - see
www.lawsons.caI couldn't find John and Isabella on the island in 1861 so I checked England and they were there:
RG9/2684, Registration district:Liverpool, Sub-registration district:Mount Pleasant , ED: 21d, Folio:81, Page:26, Household schedule number:140
Address: 8 Alexandria Place
John Gates, Head, Mar, 36, Joiner, Cumberland, Keswick
Isabella Gates, Wife, Mar, 35, Isle of Man
Fanny, Dau, 18, Whitehaven
Mary A, Dau, 10, Whitehaven
William E/C?, Son, 8, Lancashire, Liverpool
John, Son, 6, Lancashire, Liverpool
Elizabeth, Dau, 4, Lancashire, Liverpool
Henry, Son, 2, Lancashire, Liverpool
In 1871 they are living in West Derby:
RG10/3845, Registration district:West Derby, Sub-registration district:West Derby, ED:35, Folio:26, Page:1-2, Household schedule number:8
Address: 11 Albert Street
John Gates, Head, Mar, 45, Joiner, Cumberland, Keswick
Isabella Gates, Wife, Mar, 44, Isle of Man, Rushen
William E, Son, 18, Liverpool, Joiner's apprentice
John, Son, 16, Liverpool, Cabinet maker's apprentice
Elizabeth, Dau, 14, Liverpool, Scholar
Robert H, Son, 12, Liverpool, Scholar
Arthur, Son, 8, Liverpool, Scholar
From that info I would guess that this IGI entry is a likely candidate for Isabella:
Christening: 21 AUG 1825 Kirk Rushen, Isle Of Man
Father: RICHD FARGHER
Mother: ISAB HARRISON
But that is just an assumption. I would get Brian to do a check on the 1841 census for any Isabelle Faragher born abt. 1826 and also to check for the Richard Faragher/Isabella Harrison family.
Hope this helps.
Cheers,
Ruth