Hi Boland
ScotlandsPeople should have every civil registration for BDMs since 1855, but online you only get access to births to 1904, marriages 1929 and deaths 1954. Thus said, if your folks are on the 1861 census they probably died before 1954 so should be there.
As Prue suggested, they could well have moved (perhaps gone to live with children?) so you may need to search a little wider and, of course, be inventive with your names.
The IGI is mainly pre civil registration data and contains very few deaths (burials). It is mainly births and marriages.
Trish