I also have graves with no headstones. Whilst on my visit to IOM last year the Manx museum had a BIG book for Malew graveyard plots which I looked through to find the burial plot of an ancestor. It can also say who bought the plot for the dead person. You need to look at the plot number then look up the MI book to see who was buried in the adjoining graves as that makes it easier to locate the unmarked grave.