O'Neill was born in Tralibane (also Trawlebane), near Bantry, County Cork. At an early age he heard the music of local musicians, among them Peter Hagarty, Cormac Murphy and Timothy Dowling. At the age of 16, he became a cabin boy on an English merchant vessel. On a voyage to New York, he met Anna Rogers, a young emigrant whom he later married in Bloomington, Illinois. The O'Neills moved to Chicago, and in 1873 O'Neill became a Chicago policeman. He rose through the ranks quickly, eventually succeeding Joseph Kipley as the Chief of Police from 1901 to 1905. He had the rare distinction, in a time when political "pull" counted for more than competence, of being re-appointed three times to the position by two different mayors.