The casualty, who is from west Limerick, was resuscitated by paramedics after he was taken from the river near Shannon Bridge.
Firefighter Kevin Buckley, along with 11 colleagues, attended in three fire tenders when the alarm was raised at 2.45pm that a man was seen in the river near the bridge.
A strong incoming tide had swept the man away.
Firefighter Buckley dived in and swam over 30 metres to get to the stricken victim.
He and a colleague had donned swift water-rescue gear in the fire tender as they rushed to the scene from their Mulgrave Street base.
Buckley also carried a personal flotation device to help keep the man he was rescuing afloat.
Station officer James Kerans said that Buckley managed to get the man to the quay wall, where a ladder was in place to help take him from the water.
Other firefighters then administered CPR to the man before ambulance medical technicians arrived on the scene and brought him to the Mid-Western Regional Hospital in Limerick.
Scores of passersby stopped to watch the dramatic rescue.