DonRoberto is correct Irishman.

In the book Vito was shot by some Irish gang and Sonny became acting Don and successfully fought the war.

If you remember, in the movie when Michael talks to Sonny after the Don is shot, Michael asks Sonny about his mother, and Sonny says that "she's been through it before."

That's a reference to the incident in the book when Vito was shot by the irish gang.

And his being shot is what caused him to talk in that gruff voice of his.