in Life on Mars: Season 1, Grey's Anatomy: Season 4, Criminal Minds: Season 2

O'Mara was born and raised in Sandycove in south Dublin, Ireland. He was planning on becoming a professional rugby player until an injury forced him to focus his attention elsewhere. He graduated from Trinity College, Dublin with a degree in drama.

He married American actress Paige Turco in a Roman Catholic ceremony in Old Saybrook, Connecticut, on September 2003; they have one child. O'Mara became an American citizen on January 26, 2009. He performed with The Royal Shakespeare Company.

His theatre work in London and Dublin included The Jew of Malta and Popcorn, and he was nominated for Best Supporting Actor in 2002 at the Irish Theatre Awards for his portrayal of John in Neil LaBute's Bash. He appeared in Harold Pinter's The Homecoming in London and Dublin as well as Lincoln Center in New York. He has also starred in several other series, including The Agency, Band of Brothers, Monarch of the Glen, High Stakes, Playing the Field, The Bill, Berkeley Square, and Reach for the Moon.

Recently, he has starred as lead character Sam Tyler in the U.S. remake of the British series Life on Mars. He currently plays Jim Shannon, the lead role in the 2011 Fox TV series Terra Nova. O'Mara also guest-starred in the Criminal Minds season two episode "The Last Word" as the Mill Creek Killer, a serial killer who is in competition with the Hollow Man, a serial shooter played by Scott Michael Morgan.

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