They're known for pushing the limits - but now adventurers David Burns and Maghnus Collins are embarking on their greatest challenge yet.
The daredevil pair plan to swim around the country - a 1,400km journey - in less than four months. Yesterday, they set off from Coliemore Harbour in Dalkey.
David and Magnus have already succeeded in mammoth tasks, including a 250km run across the Sahara Desert and 25 high-altitude marathons in 26 days across the Tibetan Plateau. Their exploits have raised more than €100,000 for charity.
However, David said this swim will be their toughest adventure.
"Swimming as a means of covering long distances has been a largely unexplored field of human endurance. And in attempting this circumnavigation we will be stepping somewhat into the unknown - this fact excites our whole team," he said.
"This will be our single greatest physical test."
The men plan on swimming around the coast of Ireland in less than 100 days by spending up to 12 hours a day in the water, divided into two six-hour shifts.
It's all in aid of charities Gorta and the Royal National Lifeboat Institution.
Magnus admits they are risking their lives for the sake of the gruelling expedition, but said they have the expert help of expedition leader Philip Hatton and in-water support from Leisha McPartland.
"Spending up to 12 hours in the water each day and through the night presents a significant issue in terms of maintaining a stable body temperature," he added.
"Recent long-distance open-water swimming expeditions have experienced extraordinary difficulties with fat and muscle loss.
"A recent expedition saw the swimmer lose in excess of 35pc of his bodyweight," he said.