Hi, There is no stone with the surname McGrannaghan in the Old Kilpatrick churchyard.
It may be one of the stones that are laying face down and could not be recorded or there was no stone for it to be recorded. The cemetery is next to the churchyard, so they may be buried in there. You could write to West Dunbartonshire Council Clydebank and ask them if your ancestor in in the graveyard.
John Mac