Who is Mr. Harranby from The Great Train Robbery and what is their importance?

Mr. Harranby is a police detective. He becomes interested in Pierce when Clean Willy tells the police about Pierce's plan. Harranby has some of his detectives follow Pierce for a while, but Pierce manages to throw them off his path by convincing another snitch that the crime is to take place in Greenwich, not on the train. In the end, Harranby arrests Pierce, but he never learns the information he wants to know.