I would not take the different spellings of Corbett as not being your people.
Enumerators spelt names whichever way they wanted to as lots of people couldn't read or write.
The name on the headstone is Corert as you say but it also looks like it is handcarved. Maybe they just didn't have enough room! Looks like somebody mistook the B for an R.
Regardless I think it is Ann Corbett who married James Farrell.