good movie but puzzling - Irish mobster with Italian name

Rating: 8/10 (25 April 2007)

No one has been able to tell me why the head of the Irish mob is given not only an Italian name, but the name of an actual famous Italian mobster. The real Frank Costello's style, personality and methods have nothing to do with the character played by Jack Nicholson or Whitey Bolger on whom the character is supposed to be based. Can someone please enlighten me?
Throughout the movie it is clear that the lead character, Francis Costello is supposed to be Irish Catholic. Yet, there are no clues I can find as to why has the Itaian name "Costello." I have asked many intelligent people who have seen the movie, but none of them seems to have any more of a clue than I have.