Who is Buck Halloran? Who is Harper? Who is Mike a illy?

Mike Scully is leader of the Democrats.

"He was introduced by his friend to an Irishman named “Buck” Halloran, who was a political “worker” and on the inside of things."

"And just at that juncture it happened that Jurgis was introduced to a man named Harper whom he recognized as the night watchman at Brown’s, who had been instrumental in making him an American citizen, the first year of his arrival at the yards."