Hi Niccorr - Your info has helped me sort out some of the detail I have. These McGarrys/Magees/McCanns are properly entwined!! Are you ready for this?
Mary Jane McGarry (your step granny) was a niece of Robert McGarry (your step great grandfather).
Your real granny's brother John McCann married (22/10/1908) a Margaret McGarry who was a daughter of Robert McGarry (from his second marriage).
John McGarry (married to Bridget Coney), is Robert's brother.
Now looking for the McAreavey link, I have male McGarry's marrying femaie McAreavey's, but not the other way round.
Michael