So are you looking for what happened to Mary? Since a lot of the family
is at that address in 1880, including Mary's mother, if I have the family right.
Could it be that the information on Bernard's death record is incorrect, e.g, they asked his son "Was he married?" which got a "yes" answer? Otherwise Mary died in that 3 month period and should be findable. Probably buried in the same cemetery. I don't know how cooperative the cemetery will be for providing information.
She didn't go far, if her mother and children are at the same address.
If this is the family in 1870
NYC, 5th Election District, 13th ward
Bernard, 45, clerk in a store
Mary 42
Mary 19, dress maker
Kate 18, dress maker
Anne 17, unreadable maker
Thomas 15, errand boy
Lizzy 11
Margaret 9
John 4
https://www.familysearch.org/ark:/61903/1:1:M8F3-BYZThen this is probably part of the family in 1880:
NYC
239 Avenue B
[click back a page to get the street address for the Dolan family]
Thomas 23, head, wood carver
Anne, 24, sister, tailoress
Elizabeth, 21, sister, keeping house
Margaret 19 sister, tailorerss
John 14, brother, rworks in a store (fancy goods)
Eliza Brennen, 80, grandmother, born in Ireland, no occupation. disability column: sick from a beating
all born in Ireland except John, born in NY
https://www.familysearch.org/ark:/61903/1:1:MZ62-4VR--------
Is this Thomas' death?
27 September 1888, in Manhattan, parents Bernard and Mary Dolan, age 30, single, a merchant, buried 29 September 1888 in NYC
didn't give the name of the cemetery.
https://www.familysearch.org/ark:/61903/1:1:2W8V-R2D