(written from a Production point of view)
He has had many recurring roles in a variety of other television shows, such as NYPD Blue (with Gordon Clapp, Sharon Lawrence, John Schuck, Titus Welliver, and Willie Garson), Murder One (with Daniel Benzali, John Fleck, Barbara Bosson, Gregory Itzin, Donna Murphy, Natalia Nogulich, Anne Haney), Party of Five (with Rebecca McFarland, David Selburg, and Frank Novak), Buffy the Vampire Slayer, 24 (with Zachary Quinto and Greg Ellis), and Desperate Housewives (with Teri Hatcher, Steven Culp, Mark Moses, Brenda Strong, and Bob Gunton).
He has made guest appearances on 21 Jump Street (with Margot Rose), ER (with Lily Mariye, Ed Lauter, and Penny Johnson), Brooklyn South (with Titus Welliver, James B. Sikking, and Richard McGonagle), The Practice (with William Shatner, Stan Ivar, and Jim Jansen), Boston Legal (with William Shatner, Rene Auberjonois, and Henry Gibson), The Nine (with John Billingsley and Bonita Friedericy), Cold Case (in which he and Nicholas Coster played younger and older versions of the same character; the episode also featured Leon Russom, William Lucking), and Prison Break (with Wade Williams, Robert Knepper).
He also appeared in the acclaimed HBO mini-series From the Earth to the Moon playing astronaut Jim McDivitt. Others who appeared in the mini series are Sam Anderson, David Andrews, Dan Butler, David Clennon, Ronny Cox, Brett Cullen, J. Downing, Ann Cusack, David Drew Gallagher, Jerry Hardin, Clint Howard, Daniel Hugh Kelly, John Carroll Lynch, Dakin Matthews, Kieran Mulroney, Holmes R. Osborne, Harve Presnell, Mark Rolston, Stephen Root, and Alan Ruck.
More recently he can be seen on CSI (along with Wallace Langham and Liz Vassey) as the corrupt Undersheriff Jeffrey McKeen, who later murders CSI Warrick Brown. Prior to CSI, O'Farrell played the notorious child-molester/serial-killer Graham Spaulding on Without A Trace (starring Enrique Murciano). In the first season, he appeared in an episode with Kathleen Garrett and an episode with Spencer Garrett.