However, he is optimistic about medical test results due today and is looking forward “with giddy excitement” to a series of concerts in May at his ancestral home of Slane Castle.
The 63-year-old grandfather was diagnosed with lung cancer in 2013, and had the lower lobe of his right lung removed, followed by three and a half months of chemotherapy and six weeks of radiation.
“I did feel pretty suicidal,” he told John Murray on RTÉ Radio yesterday. “With chemo, it affects everyone differently, how they deal with it is different, and one thing I say to people who are going through it, or have gone through it, is to keep reminding themselves it’s toxic stuff they’re putting into you.
“So if you go into a tunnel keep telling yourself it’s the drugs that are doing it. It’s not where you are.”
He described the depression he felt during his treatment as “pretty savage” and spoke about how he considered taking his own life.
He said: “That deep darkness was pretty savage at times and I hate to admit it but I did feel pretty suicidal.”
After receiving one all-clear, he is optimistic about the results of another CAT scan today.
He told Murray: “What in a sense was wonderful about it was coming out of it, just like looking at this ray of sunlight you knew was there.
“The frightening thing was not always being able to see it.
“I’ve had one all-clear and I’m hoping to God tomorrow that I’ve jumped the next fence.
“I’m optimistic. It’s not easy, and it’s not easy for your family.
“No man is an island, and what happens to you affects other people.”
Lord Henry paid tribute to his wife, Iona Charlotte, for helping him through his illness.
“It’s been very tough on my wife, who has been brilliant, and without her I couldn’t have got through it,” he said.
“My kids have been wonderful... they know the result is coming tomorrow and they’re waiting, in a sense, probably with as much anxiety as me, maybe even more so than I am because I have to live with this and deal with it.”
The cancer was first diagnosed when Mountcharles was in the US in January 2014 and ended up in the ER in Florida for a kidney stone problem.
“I was put through a cat scan to locate the stone and a sharp-eyed radiographer saw something on my right lung that she didn’t like the look of,” he said.
“I was completely unaware that this problem was brewing so by God I bless that kidney stone to this day.”
Having taken a break from Slane last year, he is looking forward to getting back into the music business for which he is famous.
He has hosted a very successful series of rock concerts since 1981, held in the grounds of Slane Castle.
This time around, he will welcome headline act Foo Fighters to Slane.
Also playing there will be Kaiser Chiefs, The Strypes, and Hozier on Saturday, May 30.
He said: “I have a sort of lightness in my step. At times I feel almost a little giddy,” he said.
“I look forward to things with a more acute sense of excitement. It’s a complete alteration of what is important in life.
He is particularly excited about Foo Fighters, founded in Seattle in 1994 by former Nirvana drummer Dave Grohl.
The band released their eighth studio album, Sonic Highways, to critical acclaim last November.
“I wanted to go back to the roots and what better way to do that than with a guy who started off his music career as the drummer in Nirvana,” said Mountcharles.