He has written biographies of U.S. President Richard Nixon, Senator Ted Kennedy, House Speaker Thomas "Tip" O'Neill, and defense attorney Clarence Darrow.

Born in Huntington, New York, Farrell graduated from the University of Virginia in 1975[citation needed] before working at newspapers in Montgomery County, Maryland, Annapolis and Baltimore.

Excerpts of his work have been published in Jack Beatty's collection Pols: Great Writers on American Politicians from Bryan to Reagan, and in Leadership for the Public Service by Richard A. Loverd.

Farrell was a contributor, as well, to The Boston Globe's 2004 biography of United States Senator from Massachusetts John Kerry.

Farrell is an on-camera commentator in the PBS American Experience documentaries "Jimmy Carter" and "The Perfect Crime," a study of the Leopold and Loeb thrill-killers case, and in the television series The Irish in America.