Stan, my ggrandfather, John Gallagher, was in Ny - Staten Island, for the 1880 census. Two doors down was a Thomas Gallagher. Since Thomas was the name of John's father, this Thomas could have been his brother. John named his children some of the same names as your family, including Michael and Mamie.
Do you have any more information about your Thomas? MY e-mail address is firstname.lastname@example.org