Her maiden name is given as Brannan about as often as Brennan.
Sources
"Massachusetts Births, 1841-1915," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:FXCJ-BBN : 11 March 2018), Mary Frances Brannon, 07 Aug 1856, Stoughton, Massachusetts; citing reference ID #p304 No128, Massachusetts Archives, Boston; FHL microfilm 1,420,935.
"United States Census, 1870," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MDQ7-3PS : 12 April 2016), Mary F Brennan in household of Patrick Brennan, Massachusetts, United States; citing p. 70, family 632, NARA microfilm publication M593 (Washington D.C.: National Archives and Records Administration, n.d.); FHL microfilm 552,135.
"United States Census, 1880," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MHXZ-FZP : 12 August 2017), Mary F Brennen in household of Patrick Brennan, Stoughton, Norfolk, Massachusetts, United States; citing enumeration district ED 530, sheet 268A, NARA microfilm publication T9 (Washington D.C.: National Archives and Records Administration, n.d.), roll 0549; FHL microfilm 1,254,549.
Massachusetts, Marriage Records, 1840-1915
"Massachusetts Town Records, ca. 1638-1961," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:FHV7-Z81 : 4 November 2017), Andrew Edmund Hayes and Mary Frances Brennan, 05 Aug 1880; citing Randolph, Norfolk, Massachusetts, United States, Town clerks and local churches; FHL microfilm 1,954,642.
"United States Census, 1900," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M9YC-ZKN : accessed 8 October 2018), Mary F Hayes in household of Patrick Brennan, Stoughton Town (east part), Norfolk, Massachusetts, United States; citing enumeration district (ED) 1074, sheet 24A, family 565, NARA microfilm publication T623 (Washington, D.C.: National Archives and Records Administration, 1972.); FHL microfilm 1,240,672.
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