Early life

Cahill was born in New York City, New York. He is the middle of three children with one older and one younger sister. He is of Irish descent from his father, a stockbroker, and of Italian descent from his mother, an elementary school teacher.

Cahill graduated from Byram Hills High School in Armonk, New York in 1996. He attended Skidmore College in Saratoga Springs, New York and the Atlantic Theater Acting School, part of the Tisch School of the Arts at NYU.

Career

In 2000 Cahill performed in Nicky Silver's Off-Broadway production of The Altruists. Shortly after, he made several television guest-star appearances including Sex and the City, Felicity, and Law & Order: Special Victims Unit. Cahill was also a recurring guest-star in the NBC sitcom Friends as Rachel's young assistant and boyfriend, Tag Jones. In 2002, he starred in a short-lived WB drama, Glory Days.

In the 2004 hockey movie Miracle, Cahill had the chance to play his boyhood hero, goalie Jim Craig. He had never played the goaltender position prior to the movie, so most of the game-action sequences of Craig were filmed with former NHL goalie Bill Ranford doubling for Cahill, although Cahill did shoot several key sequences from within goal.

When CBS decided in 2004 to create a third CSI series, CSI: NY, to add to its franchise, Cahill was hired to play the cocky, snarky homicide Detective Don Flack who backs up the team of CSIs and who gets some of the best lines. The show ran for nine seasons and he was in all 197 episodes.

Cahill went back to the Atlantic Theater Company in June 2011 and performed at the conclusion of their 25th Anniversary season. He was in Tom Donaghy's one-act play, I Need a Quote, about "a hilarious telephone conversation between a single mother and a home insurance salesman."

In the summer of 2012 Cahill performed in David Adjmi's play, 3C, at the Rattlestick Playwrights Theater in New York City. He played "annoying neighbor Terry, a swinging bachelor true to the era, who unlike everyone else in this play, has no deep feelings at all."

In 2014 Cahill was cast as a main character starting in the second season of the CBS summer drama Under the Dome working with former CSI: NY actresses Natalie Martinez and Rachelle Lefevre. He played Sam Verdreaux, a former EMT and reclusive brother-in-law to "Big Jim" Rennie. The series was canceled after three seasons.

Cahill was cast in the upcoming 2016 ABC legal drama Conviction. He plays Manhattan District Attorney Wayne Wallis who creates the Conviction Integrity Unit, which was set up to reexamine cases where there is a credible suspicion of wrongful conviction.

Personal life

Cahill married his longtime girlfriend Nikki Uberti in Los Angeles on July 12, 2009. Uberti is a makeup artist, former model, and ex-wife of photographer Terry Richardson. The couple welcomed a son in 2009.

On his right shoulder Cahill has a tattoo of his wife's name in a heart with an arrow through it and a swallow on top.

Cahill is an avid New York Rangers hockey fan. He even wrote an NHL Celeb Blog for three seasons. He is also a fan of The Celtic Football Club.

Work

Year	Title	Role	Notes
Stage
1999	Gramercy Park is Closed to the Public	Dex	Powerhouse Theater
2000	The Altruists	Lance	Vineyard Theatre
2011	I Need a Quote	Salesman	Atlantic Stage 2
2012	3C	Terry	Rattlestick Playwrights Theater
Television
2000	Sex and the City	Sean	Episode 3.04 "Boy, Girl, Boy, Girl..."
2000-01	Friends	Tag Jones	Episodes 7.04 "The One with Rachel's Assistant", 7.05 "The One with the Engagement Picture", 7.08 "The One Where Chandler Doesn't Like Dogs", 7.09 "The One with All the Candy", 7.12 "The One Where They're Up All Night", 7.14 "The One Where They All Turn Thirty", and 8.02 "The One with the Red Sweater"
2000	Charmed	Sean	Episode 3.05 "Sight Unseen"
2000	Felicity	James	Episodes 3.09 "James and the Giant Piece", 3.10 "Let's Get It On" aka "Final Touches", and 3.11 "And to All a Good Night"
2001	Law & Order: SVU	Tommy Dowd	Episode 2.17 "Folly"
2002	Glory Days	Mike Dolan	All nine episodes
2002	Haunted	Nicholas Trenton	Episodes 1.07 "A Three-Hour Tour" and 1.09 "Simon Redux"
2002	Dawson's Creek	Max Winter	Episode 6.09 "Everything Put Together Falls Apart"
2004-13	CSI: NY	Det. Don Flack	All 197 episodes
2014-15	Under the Dome	Sam Verdreaux	26 episodes (2.01-3.13)
2016-	Conviction	Wayne Wallis	Main cast (upcoming)
Film
2004	Miracle	Jim Craig	Bill Ranford doubled for most of the on-ice action.
2005	Lords of Dogtown	Larry Gordon
2008	This Is Not a Test	Robert Forte	Cahill worked with CSI: NY castmates Hill Harper and Carmine Giovinazzo.
2008	The Narrows	Nicky Shades	Cahill was burned when a special effect went wrong.
Other
2004	The Making of "Miracle"	Himself	Video short documentary
2004	From Hockey to Hollywood: Actors' Journeys	Himself	Video short documentary
2008	CSI: NY - The Game	Det. Don Flack	Video game (voice only)

[ Source: Wikipedia ]