On May 17, 1910, he and Marty Brennan entered a saloon where Lennon was working. After another argument between the two, Lennon drew a revolver and shot Gallagher in the stomach (two of these bullets were still lodged in his body at the time of his trial). Gallagher and Brennan entered another saloon at Eleventh Avenue and Forty-Fifth Street where, according to Gallagher, they unexpectedly encountered Lennon who was now working there. Upon spotting the two gang members, he swore at them and apparently went for his hip pocket when Gallagher pulled his pistol and fired three shots, killing him. He and Brennan left the saloon and were arrested shortly after.