The How I Met Your Mother star plucked Erin Evans out of the audience at the event in Dublin's Savoy yesterday (June 13), before welcoming on stage her partner Stephen Lawlor, who popped the question in front of those in attendance.
The grand gesture, organised along with Dublin's 98FM station, succeeded and Evans agreed to become Lawlor's wife.
Speaking with the happy couple afterwards, Segel joked: "Now you're gonna have to pay to see the movie."
The Five-Year Engagement stars Segel and Emily Blunt as a couple whose relationship becomes strained when their engagement is continually extended.
The film, which Segel previously described as "a challenge to write", will hit UK cinemas on June 22.
Watch a trailer for The Five-Year Engagement below: