Here is a possible arrival of Margaret with another person, may be her sister Ellen, in 1879. They are listed as "servants"
There are other arrivals listed for other years but none show a middle name so it is impossible to be certain.
Name: Margt McMahon
Estimated birth year: abt 1859
Age: 20
Port of Departure: London
Port of Arrival: Sydney, New South Wales
Voyage Arrival Date: 6 May 1879
Vessel Name: Peterborough
Richard