Irwin made his feature debut as Ham Gravy, an old beau of Olive Oyl, in Robert Altman's "Popeye" (1980). In the early 1980s, he received numerous grants including the prestigious MacArthur fellowship, which supported him for five years as he expanded his various talents. This included co-writing, directing and starring in the Broadway show "The Regard of Flight" (1987), a comic showcase; writing, directing and starring in the off-off-Broadway drama "The Court Room"; and appearing alongside Robin Williams, Steve Martin and F. Murray Abraham in the Mike Nichols-directed 1988 revival of Beckett's "Waiting for Godot" as the almost silent Lucky.

Irwin's film and TV roles—mostly small but memorable turns—have not yet provided comparable showcases for his prodigious talents. His feature roles include Eddie Collins, a member of the Chicago "Black" Sox, in John Sayles' "Eight Men Out" (1988), Rick Moranis' FBI partner in "My Blue Heaven" (1990), a mime who taunts Woody Allen in "Scenes From a Mall" and Charlie Sheen's ill-fated father in "Hot Shots!" (both 1991). He received his widest exposure on the series "Northern Exposure" as the mostly silent Flying Man, a circus performer and would-be boyfriend of Marilyn Whirlwind. Irwin returned to the Broadway stage with fellow clown David Shiner in the uproarious silent comedy "Fool Moon" (1993 and 1995 and 1998), for which he won a Tony Award in 1999.

Irwin next had a small role in Sam Shepard’s mannered western, “Silent Tongue” (1993), then put in appearances in episodes of “Dave’s World” (CBS, 1993-1997) and “3rd Rock from the Sun” (NBC, 1995-2001). He spent the remainder of the 1990s doing a variety of theater, including the Public Theater’s production of Samuel Beckett’s “Texts for Nothing.” He also played Trinculo in “The Tempest” starring Patrick Stewart, Galy Gay in Bertolt Brecht’s “A Man’s a Man” and Medvedenko in Chekhov’s “The Seagull.” He returned to features with a small role in the period comedy “Illuminata” (1999), then was Tom Snout in a modern take on The Bard’s “A Midsummer Night’s Dream” (1999). After a small part in the indie romantic comedy “Just the Ticket” (1999), Irwin was the father of darling Cindy Lou Who in Ron Howard’s ADD-inducing “Dr. Seuss’ How The Grinch Stole Christmas” (2000).

Irwin appeared in HBO’s “The Laramie Project” (2002), a docudrama focusing on the trial and reaction of the brutal murder of gay college student Matthew Shepard. After appearing in “The Guys” with Sigourney Weaver and “The Goat, or Who Is Sylvia” with Sally Field—both in 2002—Irwin appeared as a barking drill instructor who tries to shape up an angry, rebellious seventeen-year-old (Kieran Culkin) in “Igby Goes Down” (2002). In 2003, he wrote and performed “Harlequin Studies” for the Signature Theater Company in New York, a commedia dell’arte featuring Irwin reinventing himself into different clowns by repeatedly re-emerging from an oversized trunk, once again earning the talent artist rave reviews.

A small role as a scoutmaster in Jonathan Demme’s remake of the classic psychological thriller, “The Manchurian Candidate” (2004) was followed by a Tony Award-winning performance as George—chief foil and favorite punching bag of the drunken, slovenly Martha (Kathleen Turner)—in a Broadway production of Edward Albee’s “Who’s Afraid of Virginia Woolf?” Irwin then costarred in M. Night Shyamalan’s much-maligned “Lady in the Water” (2006), playing a bookish shut-in who barely speaks to his fellow apartment tenants as they try to help their superintendent (Paul Giamatti) get a mysterious water nymph (Bryce Dallas Howard) back to her world before she’s killed by evil creatures out to get her.