The RNLI is appealing to paddle boarders and others using the water for recreational activities to wear a lifejacket.
It comes as a paddle boarder was rescued 150 metres from shore at Blackrock in Dublin.
The RNLI unit at Dún Laoghaire was launched to the scene just after 6.30pm last night after a member of the public contacted the Coast Guard.
The RNLI said that the conditions at the time “were quite rough with a squall causing strong offshore wind gusts, along with a changing outward tide and choppy waters”.
The lifeboat crew found the casualty exhausted having tried to paddle and swim back to shore.
“The crew quickly came alongside and brought the person on board,” a statement said.
“They then carried out a casualty care assessment and observed that the casualty was showing signs of hyperthermia due to a long period in the cold sea.
The RNLI said that the casualty was taken back to shore and handed over to a waiting National Ambulance service crew for further medical care.
“The crew and I are very happy with the outcome of this evening’s callout having safely returned the casualty to shore and into the care of the National Ambulance service,” said Alan Keville, Dún Laoghaire RNLI's Inshore lifeboat Helm.
“Please if anyone is planning to enter the water for any kind of recreation to always wear a lifejacket or PFD (Personal Floatation Device) and to carry a reliable means of calling for help should the worst arise.
The Coast Guard added in a statement: "Without the member of the public seeing this person, they may not be here now. Please make sure to wear a life jacket at all times on the water, check the weather and seek some training or guidance in the use of the equipment your using."
It was a busy day for the RNLI, with the Baltimore crew in West Cork called out for two rescues.
The first was to help two people on a yacht in Baltimore harbour, while the second saw a crew provide a medical evacuation in the early hours of Monday from Sherkin Island.