Here are two sources with a Mary McGrann born in NJ about 1909.
"United States Census, 1930," database with images, FamilySearch(https://familysearch.org/ark:/61903/1:1:X48S-YT1 : accessed 15 December 2017), Mary Mcgrann in household of Queenie Mcgrann, Atlantic City, Atlantic, New Jersey, United States; citing enumeration district (ED) ED 31, sheet 4A, line 29, family 88, NARA microfilm publication T626 (Washington D.C.: National Archives and Records Administration, 2002), roll 1309; FHL microfilm 2,341,044.
"United States Census, 1940," database with images, FamilySearch(https://familysearch.org/ark:/61903/1:1:K71T-RF4 : accessed 15 December 2017), Mary E Mcgrann in household of Helen E Dickinson, Milford Town, New Haven, Connecticut, United States; citing enumeration district (ED) 5-124A, sheet 62A, line 8, family 205, Sixteenth Census of the United States, 1940, NARA digital publication T627. Records of the Bureau of the Census, 1790 - 2007, RG 29. Washington, D.C.: National Archives and Records Administration, 2012, roll 518.
However, I think you may be making a lot of work for yourself. Have you been in touch with St. Peter's, New Brunswick? If Francis lived in the same area for most of his life, there could be a number of records held by the church on him.
Keep in mind, there is no guarantee that Voultaine is Myrtle's maiden name. Her mother may have remarried to that name after Myrtle was born. Good luck with your research! -NGP