During his time in St Vincent's Hospital, he lost all his body hair, his weight dropped by four stone and his waistline dropped to 34 inches.
Not surprisingly, family and friends, including RTE stars Pat Kenny and Derek Mooney regularly paid him visits.
Speaking to this week's RTE Guide, Dermot, who left hospital in the past month, said: "Six months ago I really thought I'd be dead by now."
Last summer, O'Neill visited his doctor with a suspected bleeding stomach ulcer. A week later, his worst fears were confirmed when he was told it was cancer.
Days later, Dermot went to St Vincent's Hospital on a Sunday night with his bags packed, ready to start treatment the following morning. He said: "I remember standing at the door. "There was no one around so I rang the bell. At that moment I had this overwhelming feeling that I may never come out of here again. God, I had cancer in my stomach. I went up the ward where I was met by a nurse and at that point I just exploded into tears.
"The nurse was fabulous. She threw her arms around me and said, 'It's okay to cry. Most men don't cry but it's good to let it all out.'"
Dermot spent nearly seven months in St Vincent's.
"I wasn't eating, I had lost all my sense of taste and I was getting all the side effects -- dry mouth, bad headaches, feeling sick and exhausted -- associated with chemo," he said.
"I did a lot of thinking, a lot of soul-searching. I now certainly appreciate things a lot more than I did."
Dermot is now working on a major new TV series, Clondeglass, a diary of his life since he was first diagnosed with cancer, due to air in January 2011.
He has returned as resident gardening expert on the Mooney show and his new reality series Super Garden is due in May.