The incident happened on Saturday, when the 37-year-old man from Carrigaline went overboard from a RIB near Haulbowline Island. It is understood that the man was struck by the RIB after he entered the water.
With no other boats in the area at the time, it was by a sheer stroke of luck that he was spotted from a mile away by a man looking out through a telescope at Fort Camden in Crosshaven.
A rescue effort was quickly mobilised, with the Cork Pilot boat and Crosshaven lifeboat both speeding to the scene.
The man, who was found seriously injured, was taken to the Crosshaven lifeboat station from where he was rushed to hospital.
An RNLI spokesperson said the man was "incredibly lucky to have been spotted from shore".
The injured man's family have since praised the telescope user, who is as yet unknown, for his quick thinking in ensuring the rescue of the father-of-two. The Evening Echo has more on the story HERE.
It's the second dramatic rescue in Cork in the past few weeks, coming after brave volunteers from the Baltimore RNLI halted a runaway RIB heading for a busy pier, as previously reported on Afloat.ie.