Henry O’Donnell, 56, started his marathon swim at Carrickfin beach in Co Donegal last month with the aid of fins and a wetsuit after four years of training.
The father-of-six is raising money for charities the Irish Cancer Society and Water Safety Ireland during his epic challenge.
Mr O’Donnell said he has been swimming between three to 10 miles a day, depending on conditions.
He has conquered some challenging stretches of water including Tory Island, Malin Head and the sound of Rathlin where the Atlantic Ocean meets the Irish Sea.
“Each day when I go out to swim I try to swim as far as I can but taking into account not to overdo it, so it’s different distances each day,” he said.
“I have a tracking system on and the public can track it online.”
He will now continue south and expects to arrive in the waters around Dublin in around three weeks time, with the aim of completing the swim within a year.
Mr O’Donnell said he has been warmly welcomed from the chilly waters by local people all along his route.
“The welcome has been amazing and the support of everybody,” he said.
He added: “These are two very important charities for the island of Ireland because people benefit from both.
“Two very worthy causes.”
Mr O’Donnell arrived on the north Down coast shortly after the death of sea swimmer Mary Feeney, an artist who was originally from his native Donegal.
He joined a vigil in her memory in Bangor.
“I dedicated the swim the day before yesterday to Mary Feeney, we will remember her,” he said.
Marie-Therese Davis Hanson and her husband Stephen were among members of swimming group the Brompton Belles and Beaux who greeted Mr O’Donnell at Bangor Marina.
They presented him with a club woolly hat, declaring him an honorary beaux.
“We heard about Henry’s challenge and thought it was a fantastic cause so we thought we’d come down to welcome him and share some of our knowledge, and help him out in any way we can on his swim,” Ms Davis-Hanson said.
Mr O’Donnell is no stranger to challenges, having previously become the first Irishman to swim the 14km around Tory Island, and later completed the 38km Donegal Coastal Challenge Swim in 1997.
His progress can be tracked on the challenge website.
