McDonnell was born in Galway in Ireland. He attended the Central School of Speech and Drama in London, England, moving to the city in 1996. Before breaking into film and television, McDonnell did a lot of theatre in Ireland and the United Kingdom.
McDonnell had a role in the 2003 feature film Conspiracy of Silence. It was RTÉ Television's police drama Single-Handed that was McDonnell's television career break. He starred in all four series of the programme and was nominated for an IFTA Award for the role in 2011. McDonnell was given the lead role of Garda Sergeant Jack Driscoll after receiving a call from casting director Maureen Hughes. Appearing onstage in The Lieutenant of Inishmore in Galway Town Hall at the time, McDonnell, alongside two other cast members, left for Dublin to read the script for the original Single-Handed director Colm McCarthy. He was given the lead role a day later.
In 2014 McDonnell starred as Inspector Fiachra Greene in the Irish language movie An Bronntanas (The Gift), screened at the Galway Film Fleadh and later put forward as Ireland's submission for the Best Foreign Language Film category at the 87th Academy Awards (2015). The film is edited from the five-episode television series of the same name, screened on TG4 in October and November 2014.
In 2015 McDonnell starred in the hit TG4 series An Klondike, a western set in the Klondike gold rush. In 2016 he starred in Wrecking the Rising, about time travelling gobdaws who accidentally kill PH Pearse before the Easter Rising, and in the short film An Béal Bocht based on Flann O'Brien's satirical novel.
In 2018, McDonnell first appeared as Niko Polastri, husband to Sandra Oh's title character in Killing Eve. Upon meeting him at the first script read-throughs, Oh immediately felt that their on-screen relationship would be believable, and would serve as an important grounding force for her character.
McDonnell lives in London with his British girlfriend Jill.
|2003||Conspiracy of Silence||Noel|
|2007–10||Single-Handed||Sergeant Jack Driscoll||RTÉ|
|2008||An Ranger||An Ranger||(Short film)|
|2009||Wild Decembers||Joseph Brennan||RTÉ|
|2009||Spooks (series 8)||Vadim Robinov||BBC (guest star)|
|2009||Rásaí na Gaillimhe||Eamonn O'Connell||TG4|
|2011–12||Mount Pleasant||Jack||Sky1 (series 1 and 2)|
|2011||The Wright Stuff||Himself (Guest)||Channel 5|
|2012||A Year of Greater Love||Neil||(TV film)|
|2012||Saving the Titanic||Thomas Dillon||(TV film)|
|2013||A Terrible Beauty...||Ned Daly||(TV film)|
|2013||Ghost Train||Michael||(Short film)|
|2013||Made in Belfast||Smith|
|2013||Scéal||Morris||TG4 (Short TV film)|
|2014||The Question||Dr. Fialan||(Short film)|
|2014||Silent Witness (Series 17)||Ryan Hodgson||BBC (guest star)|
|2014||An Bronntanas||Inspector Fiachra Greene||TG4 (film and TV series)|
|2014||Swung||David||Sigma Films (feature film)|
|2015–present||An Klondike||Tom Connolly||TG4 (film and TV series)|
|2016||My Mother and Other Strangers||Michael Coyne||BBC/RTÉ|
|2018||Killing Eve||Niko Polastri||BBC America|
|2018||Women on the Verge||Sean|