I have recently tentatively linked this same Michael Gannon and Mary Walsh to my family tree - maybe we can help each other.
My great grandfather (GG1) John Joseph Gannon immigrated to America in 1918, coming from Westport, Mayo. His father was (GG2) John Michael Gannon, and his mother was Bridget Ginely (spelled a few different ways depending on the document), also from Westport, Mayo. (GG2) John's father was a Michael Gannon (GG3), and I am unclear on any other details, so I've explored options available through records on Familysearch.org, and found Michael Gannon a widow, married to Mary Walsh, who together had a son John (amongst others children), so I linked this same Michael Gannon as an ancestor. The birth years and marriage year make sense with my GG2 birth year of 1868. (They were married in 1864/65).
I believe my great grandfather (GG1) brought his parents John Michael Gannon and Bridget (Ginely, or Grinely, or Ginelly) here to America once he had a residence. I suppose I assumed his grandfather (GG3) Michael, and grandmother Mary Walsh, perished in Westport, Mayo.