Hi Anne,
Welcome to RootsChat - and to a fascinating new hobby!
Here's something for starters.
The 1930 US Census has the following family living in Philapdelphia:
Michael McMahon, 26, Motorman street cars;
Julia McMahon, 23, wife;
Mary C. McMahon, 0, daughter;
Martin Maher, 50, father-in-law, Fireman [electric ?? battery?].
Mary was born in Pennsylvania, but the others were all born Irish Free State. They were 25, 21 and 28 respectively when first married. They owned their home, which was worth $4000.
Unfortunately, it is hard to read the years in which they immigrated to the State. Could be Michael 1915 or 1925, Julia 1911 or 1921, Martin 1925.
That's about all I can glean from it. Something to go on with?
All the best in your quest,
Ros