Carol Cashell, from Cork City, has set the challenge to raise funds and awareness of the RNLI, and the swim will take place this Saturday 31 August.
Having already completed the Cork-to-Cobh and Lake Zurich marathons, she has spent the winter months getting ready for the 28-mile swim around New York's Manhattan Island in June, closely followed by the 2-Way Channel Relay in July.
Cashell’s latest swim will be the 24km route around Bere Island, and this will be particularly challenging as some of the island’s coast is heavily exposed to the Atlantic ocean.
However, Cashell has plenty of relatives locally and they, along with the RNLI volunteers, will be supporting her on Saturday.
Local lifeboat fundraising secretary Sheila O’Driscoll, said: "It’s been a busy a summer for Castletownbere lifeboat and sadly there have been a number of tragedies around our coast in the last few months.
"We are delighted that Carol is doing this swim to help raise funds for our local lifeboat. Sponsorship cards are available in all the local businesses."
Speaking of the challenge, Carol Cashell commented: "I swam in Castletownbere last week. The jellyfish are much bigger than what we have in Cork Harbour at the moment - must be something in the West Cork water!"