just noted a Wm Grant was a witness to a 1781 marriage of James Tear fencible (of Castletown) +Elizth Nicholson

images of Malew Parish reg are on web (possibly via iMuseum and or Ancestry or Family search) so you may be able to check signatures

Not sure if I've photos of this period - if not online then try library@mnh.gov.im - tho at this period some of their few staff are away