Hi Jerri Thank you for this post. The problem with this information is that for Mary to be the niece of Susan and Denis they in turn would have to be the sister and brother to Sarah. The problem is that while Susan and Denis did have a sister Sarah she was not married to a James Doherty but was in fact married to a Patrick Hirrell.
So either the information is wrong or we are looking at a different family group. If they had said Mary was her cousin this would have made more sense but it looks as there is a puzzle here I can't get an answer to.