I presume the family were RC ... the records for Rathfarnham go back to the 1770s or so - so maybe they married somewhere else ?
It might be worth trying the neighbouring parishes to see if Mary's family moved outside Rathfarnham at some stage. In the 1850s these would probably be Sandyford, Saggart (includes Rathcoole & Newcastle), Clondalkin, Rathmines, Booterstown and possibly Palmerstown (which included Lucan & Clondalkin)
Shane