In some cases if you contact the Vicar/ / Warden / caretaker in advance for help in finding the exact place of a burial you may be lucky. In some parishes I understand it can be difficult.
Recently I assisted a Welsh family who wished to locate the grave [shared ] of an ancestor found washed up at Port Erin & buried as unknown at that time. The folk at Rushen Parish church made
me proud to be Manx. The caretaker met them , having already measured out a plot & showed them to it. They had brought a pot of daffodils & were allowed to plant them as of course no memorial
is ever allowed on a shared grave. Having said that there is an exception at Braddan new where the authorities erected stones on graves for inmates from the Asylum inscribing only the surnames.