The Scottish-born star, who has appeared in films such as The Man Who Sued God and The Eye of the Storm, shocked the audience at the city's Belvoir St Theatre when he stumbled across the stage and passed out during the play's second act.
A doctor in the audience got up to help Friels and the 59-year-old actor was taken to St Vincent's Hospital for tests.
A spokesperson for the theatre revealed Friels had been feeling unwell during the interval but "was very stoic and insisted on continuing with the show".
Friels, who has been battling pancreatic cancer, reportedly came down with a bout of flu earlier in the week (beg13Aug12) and is now resting at home after being discharged from hospital. Theatre bosses cancelled Friday night's (17Aug12) performance as well as two shows on Saturday (18Aug12).