He received a BAFTA Award for Performer in a Supporting Role nomination for his work in Ghost of Tsushima.

[2][3] Gallagher is known for his television roles as Joe Finn in Da Vinci's Inquest, the alcohol salesman in Entourage, Leon in The Line, Farhod the Fierce in Pair of Kings, and Ken Tanaka in Glee, and his film roles as Awkward Davies in Master and Commander: The Far Side of the World and Gary the bartender in Sideways.

He played Attila the Hun in Night at the Museum and its sequels, Chow in True Blood, Colquitt in Final Destination 3, Anderson in Severed: Forest of the Dead, and the head of security, Hugo, in the short-lived series Endgame.

Gallagher made guest appearances in Battlestar Galactica, Smallville, Hawaii Five-0, Intelligence, NCIS: Los Angeles, Fuller House and Psych in addition to Hell's Kitchen as a guest for the 100th dinner service.

He landed the recurring role of a prison guard on the ABC soap opera General Hospital.