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Cahill was born in New York City, New York. He is of Irish and Italian descent.[1] He has two sisters: one older and one younger. He graduated from Byram Hills High School in Armonk, New York in 1996.

Cahill starred on a short-lived WB drama, Glory Days (2002). His television guest star appearances include Charmed, Felicity, Sex & the City, Dawson's Creek, Haunted, and Law & Order: Special Victims Unit. He also was a recurring guest star in the comedy Friends as Rachel's hot young assistant, Tag Jones. In the movie Miracle (2004), he played hockey goalie Jim Craig.