Viv, I have most of the IOMFHS pubs of parish MIs. In the front of each there is a graveyard map showing what is where.
In the alphabetical index the grave section and number is listed for each person. Some of the older stones are damaged so the inscriptions aren't always complete.
I also have the pubs of burial registers and they often fill in some of the missing info--rest of date, rest of name, etc.
Often other members of the family are buried in adjacent plots and that can be helpful as to who belongs to who.

I do have Onchan, let me know who your seeking.

DJD