Several members of the public had reported seeing the man in difficulty and raised the alarm.
The lifeboat launched immediately and on arrival at the scene observed that a local fisherman was assisting the casualty by keeping him afloat from his 5.5m boat with the aid of a boat hook.
The crew proceeded to take the man out of the water and assess his condition before administering casualty care.
A local doctor was also present at the scene and assisted the casualty until the ambulance arrived minutes later. He was subsequently transferred to St Vincent’s Hospital for further treatment.
Commenting after the callout, Dun Laoghaire RNLI helm Gary Hayes said: “This was a tremendous effort from all involved. We would like to extend our praise to the members of the public who alerted the emergency services as quickly as they did.
“We also want to thank the local fisherman for his help in this rescue and for his efforts to saving a life.”
Locals and visitors will have a change to celebrate the lifesaving work of Dun Laoghaire’s RNLI volunteers at the lifeboat station’s open day from 1pm this Sunday 21 July.
The station at Carlisle Pier will host tours of the all-weather lifeboat Anna Livia, while the inshore lifeboat Realt Na Mara will be on display at the old boathouse beside Rogan’s Slipway on the East Pier.
Souvenirs in support of the charitable work of the RNLI will be on sale in the station shop throughout the afternoon.
And special guests will be visiting the RNLI station during the afternoon, including Stormy Sam and the Dalkey Ukulele Klub.
Stephen Wynne, Dun Laoghaire RNLI lifeboat operations manager, said: “We are delighted to welcome everyone to inspect the lifeboats and meet the volunteer crew members and get an insight to what they do!”