hello J , until 1829 all cemeteries in ireland were in theory , controlled by the church of ireland .for Sandyford that would have been the cemetery in Dundrum .The catholic cemetery for Sandyford is in Glencullen but I do not know how old it is.
Kilgobbin is an outside possibility as is Cruagh.This is all a bit academic though as the catholic church does not keep records of deaths and 1849 predates the establishment of civil records in Ireland . if they were buried in Dundrum , the church there may have records of them. There are records for the cemetery in glencullen but I do not know how far back they go.
There is a remote possibilty they were buried in Glasnevin but at that time it would have been quite a journey . Glasnevin has extensive records dating from 1832. look for Glasnevin cemetery online.
Did your peter ever fill in a census record in the US , he may have said what county he came from ?
best regards, Brian.