Shamrock Rovers winger James Doona produced an extra-tme goal to set up an EA Sports Cup final clash with Dundalk.
The substitute swivelled in the area before sending in an extra-time left footer that broke the resolve of a Cork side that had defended brilliantly with ten men for 78 minutes, after Alan Bennett's 38th minute red card.
Cork were seconds away from a shootout when Rovers finally struck to end City's ambitions of a domestic treble.
Manager John Caulfield was also sent to the stands in a fiery encounter.