Mary, there is a more recent Kerruish will for the year of death given by Sandra: KERRUISH, Robert 1798 Mau A 88 0106233 [from Brian Lawson's index]. If you haven't already thought of this, it would be well worth getting for additional information - to see whether Cashens are mentioned, whether Robert remarried, any place names given, and what happened to the other children, etc.
Maughold parish register:
Jane KERRUISH als CASHEN buried Feb 3 1787.
Robert KERRUISH buried Apr 15 1798.
Sue