www.manxnotebook.com/gazateer/gyards/cems.htm describes all known graveyards
There are reports of burials at various old keeil sites esp St Michael's on Langness for Catholic burials and possibly Lag-ny-keeily - the old Cathedral at Peel was also used though many of the burials seem to be listed under German. I'm not aware of any other significant burial sites used in period 1700 to 1900 - if Brian is aware of any I'd like to know of them, though I know of a couple of individual graves post WW2 not in consecrated ground