He is known for his roles in the RTÉ drama Single-Handed (2007–2010), the TG4 Western An Klondike (2015–2017), and the BBC spy thriller Killing Eve (2018–2022).

[1] 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.

[11] 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.