Brían Flynn O'Byrne (born 1967) is an Irish actor who works mostly in the United States.
First attracted notice for his performances in the Martin McDonagh plays The Beauty Queen of Leenane and The Lonesome West. He is known for taking on serious and dramatic roles, such as a serial killer in Frozen (for which he won a Tony Award) and a priest accused of child molestation in Doubt. O'Byrne also appeared as a priest in the film Million Dollar Baby.
In May 2007, O'Byrne was nominated for a Tony Award for his performance as Alexander Herzen in Tom Stoppard's trilogy The Coast of Utopia.