Mr O’Neill had previously battled and beaten cancer before his passing
He made his first TV appearance 40 years ago in 1982 when he part on RTÉ children’s programmes, before presenting a gardening segment on Live at 3.
Since then the Dubliner has presented on gardening within the magazine show Open House and was been a regular contributor to Pat Kenny and Derek Mooney’s radio shows as well as being involved in Irish Garden Magazine.
He was also once a star guest on the culinary show The Restaurant.
In 2009 Dermot was diagnosed with cancer. It was initially thought to be a recurrence of a previous stomach ulcer, but he was later diagnosed with stomach cancer and non-Hodgkin lymphoma.
He was treated at Beacon Hospital, Beaumont Hospital, and St. Vincent's Private Hospital and spent almost six months at St. Vincent's, where he underwent chemotherapy under the care of Prof. John Crown and had been cancer free since 2011.
After his recovery, he completed an RTÉ TV series, Dermot's Secret Garden, about his health problems and renovations of his Victorian walled garden in Clondeglass, Co. Laois.
Tom Brady, who now owns Condeglass Walled Garden, posted on social media how sad he was at new of Dermot’s death on Friday
“He was such an inspiration to so many gardeners over the years,” he said. “I remember coming home from school and watching him on ‘Live at Three’.
“His creation of Clondeglass Walled Garden was a massive hit for RTE. I’ve been so lucky to take over the garden and continue on his work. May he rest in peace.”
Hundreds of others posted tributes to Dermot, whose funeral arrangements have still to be announced.