He said he "cried and cried and cried" when he found out that he had the life-threatening condition.
And while the popular writer said he is hopeful of making a full recovery with a combination of drugs and chemotherapy, for now he remains in the danger zone.
Speaking on RTE's Today with Pat Kenny, he revealed that doctors have had to drill a hole in his head to facilitate chemotherapy.
Dermot said that, at first, he believed he was dealing with a recurrence of an ulcer that he suffered from as a teenager. Doctors at the Beacon Clinic initially agreed, but tests showed the presence of cancerous cells.
It was confirmed that he had stomach cancer and lymphoma, which Dermot said left him "horrified and shocked".
He said this started him on an emotional journey as he was referred to Beaumont Hospital for treatment.
Experts discovered that he also had cancerous cells in his spinal cerebral fluid, and the doctors decided on a tough course of treatment.
Dermot said the reality really hit in when doctors advised him they would have to make a small hole in his brain.
Naturally, he wasn't fully on board with the idea, but was offered the choice: do it or die.
"That was the moment I realised I had to hand over my trust and my life to Dr Crown," he said. The hole made it easier for doctors to administer chemotherapy.
Thankfully, the cancer has retreated somewhat and is only in his stomach.
He noted that the drugs are working and his lymphoma was "very treatable".
Dermot is now working at Clondeglass, his walled garden in the foothills of the Slieve Bloom mountains.
He has previously written a selection of books, including Dermot's Gardens and his latest effort Ross Revealed.
He is also a regularl contributor to a variety of television and radio programmes, and has served on the council of the Royal Horticultural Society of Ireland.
He is also a founder committee member of the Irish Garden Plant Society.