Christina:
Welcome to Ireland Reaching Out!
Can you clarify if she was Mary Cannon married to a Costello or Mary Costello married to a Cannon? I located a Mary Cannon married to a Costello in 1874.
See fourth record on this page for the 1874 marriage of Mary Costello and Patrick Cannon. https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1874/11246/8124228.pdf
The bride and groom lived in Greaghrevagh townland Oughteragh civil parish.
This is likely Mary's baptismal record from the subscription site Roots Ireland
Name:Mary CostelloDate of Birth:
Date of Baptism:21-Feb-1846Address:
Parish/District:OUTERAGH ROMAN CATHOLICGender:FemaleCountyCo. Leitrim
Denomination:Roman Catholic
Father:John CostelloMother:Mary MacdermottOccupation:
Roger McDonnell