Diarmuid Gavin relaxing. Photo: PA

Irish TV gardener Diarmuid Gavin - recently one of the celebrities battling to reach the Arctic Circle in ITV's 71 Degrees North - reveals to Gabrielle Fagan how the childhood trauma of witnessing the death of his younger brother in a road accident has affected his life.

Diarmuid Gavin is beaming proudly as he watches his blonde, blue-eyed daughter, Eppie, playing happily just outside his home.

The TV gardener and Chelsea Flower Show medal winner, who became a household name through co-presenting BBC Two series Home Front with Laurence Llewelyn-Bowen and is renowned for his unconventional creations, lives in a large house in an area of outstanding beauty at the foot of Sugarloaf Mountain in Wicklow, Ireland.

It's an exclusive, private estate and there's no through traffic and a low speed limit, but his five-year-old daughter is under strict instructions that while she can play on the pavements she must never to cross the roads alone.

That restrictive safeguard is at odds with the experience of her neighbourhood friends who've been enjoying that tiny step towards independence for at least a year.

"I've explained to Eppie that accidents can happen, and she's not quite old enough yet. Really, it's that her mum and I aren't quite ready for her to do it," he says quietly as he reveals just one of the consequences of a boyhood trauma that shaped his life.

Diarmuid's own happy, secure childhood, as one of five children living with his parents in a modest house in a Dublin suburb, was shattered on February 1, 1971.

On that day, aged six, he witnessed his younger brother, Conor, five, run across a road and get knocked down and killed by a car.

It was an event that indelibly scarred and "crushed" his family and had a "profound effect on me," he says recalling the tragedy which he's written about - with moving sensitivity - in his autobiography, How The Boy Next Door Turned Out.

For behind his easy-going TV persona, Diarmuid reveals himself as "emotional and inherently shy. I never wanted to write about any of this or even do an autobiography.

"I simply wanted to tell the story of a penniless gardener ending up competing successfully at Chelsea and all the highs, lows and adventures there."

He had a struggle to persuade his intensely private wife, Justine, 42, and his parents, Joan and Jack, that the publishers, and finally he himself, were convinced the story would be meaningless unless it was set in the context of his full life story.

"Although I've referred to Conor's death in the past this was an opportunity to try to describe in detail what had happened and what it meant to us as a family," he says.

"I had tears running down my face as I wrote it. It was enormously cathartic for me in a way but also so hard to do because it's a story that belongs to everyone who knew Conor. So there was a real feeling that while I could only tell it from my perspective I still had to get it as right as I could. I didn't want to disrespect any of the family's memories or cause pain."

The weekday morning his brother died began as any other. He and his brother walked hand-in-hand the short distance from their house to meet the adult who escorted school children across the road so they could get on a school bus to their convent school.

"We knew what we had to do, we'd done it dozens of time before but this time, for some reason, once over the other side Conor ran back across the road. It was totally unexpected," he says.

"Nobody could have done anything and for the innocent man driving the car, and the guy who helped us cross, it must have been totally horrific."

Diarmuid was whisked away to a neighbour's house while police and an ambulance were at the scene, and in the resulting confusion his parents were initially wrongly told it was he, not Conor, who had died.

Eventually the shocked little boy was reunited with his father and told that Conor - the brothers shared the same birthday - had died.

"When I finally walked in home that morning there was this weird thing of overwhelming joy that I was alive and overwhelming grief that Conor wasn't," he says.

"So there were these incredibly complex emotions for all of us.

"I had real guilt, totally misplaced guilt, but terrible guilt all the same wondering whether I could have done something to prevent it and also that I'd survived. I had dreams about seeing him in heaven as well as awful nightmares for years."

His parents had to battle their own grief and care for their remaining four children, Diarmuid's older brother Declan, then nine, and their two daughters, Niamh, three, and Emer, two.

The family received immense support from their local Catholic church and community, but, he says, in the manner of the more reserved, less outwardly emotional times, the deeper psychological wounds weren't explored in the way they might be today.

"People didn't talk about things or address them like they do now. It just wasn't the way," Diarmuid explains.

"There was still laughter in our home - with two little girls running about there was bound to be - and life went on, but it wasn't the same.

"There was for a long while an enveloping sadness - at one time we used to visit the grave every week as a family - and there was always a gap, a loss felt by us all but in different ways. It changed the family dynamic."

And he remarks with a wry smile: "In Ireland we do have a tendency to embrace grief and never let it go. In one way, we deal with it very well in that it's a celebration of life and a wake for people, but then it does tend to go on a bit."

Diarmuid says he withdrew into a fairly solitary dream world.

"I was a lonely boy. I'm not sure whether I was more outgoing before the accident but I was shy, lacking confidence and a dreamer afterwards. I grew up not really knowing how I fitted into life's grand plan."

The coincidence of his and Conor's shared birthday gave added poignancy not just to that annual celebration but other events in his life.

"I think one occasion sums it up for me. It might have been a year or so after he died, and I'd just had the ceremony of my Holy Communion, a religious milestone and so special for my parents.

"Afterwards, they sat me down at the kitchen table and gave me a gift of this new, expensive watch. Then they just broke down into a hell of grief that was not of this world really. I think perhaps it was because everything to do with me was wrapped up with Conor too. I just had to leave them and remember cycling around alone on my bike."

Sometimes haunted by his memories, Diarmuid would creep upstairs alone to his father's study.

He'd seek out carefully stored press cuttings about the accident and the memorial cards sent by family and friends, and a hand-carved cross, made by a work colleague of his father, that at one time marked his brother's grave.

One of the sympathy cards to his parents, he says, said 'remember your other four children' and that was what his parents did.

He's full of praise for his parents saying he received nothing but "unconditional love from them" and partly because of what he'd gone through, he says: "I always felt a little special and appreciated at home especially in my formative years."

It's an enormous relief to him that the family received the book well.

"My dad's kind of tickled by it and very happy with it, but my mum says she won't read it yet because she's got two biographies of geisha girls to get through first!," he says hooting with laughter.

"My youngest sister, Emer texted me as soon as she'd finished the book telling me she'd been in tears reading it. I asked her if it rang true with her memories and she replied 'if it makes sense, you have a completely different take on our childhood.'

"And that did make sense, because my sisters were so young when it happened they didn't have the memories and the strains on their growing up that I did."

Healing took years, he says, and it was around 15 years before "laughter had truly returned to the family. I mean real laughter where you're laughing and not checking yourself and thinking 'why am I laughing... is it ok to laugh?'"

He's aware that his parents have reviewed the past and "beat themselves up that they might have been bad parents. They shouldn't because they weren't at all."

And he recalls: "When I was about seven they were so concerned in the change in them that they sent me a psychologist. But the doctor and I both agreed that there was nothing wrong with me, and that probably my parents had their own problems (with grief).

"My way of coping was withdrawal and disobedience. It also probably didn't help that I was different from the rest of the family and wasn't at all interested in studying, much to my parents dismay. I just wanted to be creative and grow stuff."

Gardening turned out to be the solace that helped him ease his grief and inner turmoil and finally led to his future career.

After leaving school and studying at the Botanical Gardens in Dublin, he set up his own gardening business.

In his book, with dry humour, he describes his often roller-coaster ride to fame and fulfilment which has included competing at the pinnacle of horticulture, the Chelsea Flower Show, six times.

His TV potential was spotted at his second show in 1995, when he won a bronze medal, and was interviewed by Alan Titchmarsh, and he raised eyebrows with a 2004 Chelsea garden exhibit featuring lottery balls on poles that looked like large lollipops, but it won him a silver medal.

He justifies his fiery outbursts and spats with authority, and sometimes other gardeners, as usually caused by "my passion for gardening and a stubborn refusal to accept mediocrity." But, he acknowledges, "few people know how to handle me."

One person who does is his wife, Justine, daughter of the former Chief Justice of Ireland, Ronan Keane. They've been married 15 years, and he says: "She's eloquent and elegant and completely supportive and has given me manners in that she's shown me you have to step back and consider that if you create a situation you need to know how to deal with the consequences."

But he admits that Justine, who shuns the limelight, doesn't welcome the "media storm when I make a commotion." Then his blue eyes twinkle as he says: "But she knew the person she married. I haven't changed from that day to this. I think she just thinks 'oh no, he's done it again!"

Their daughter is their focus and "of course we're hesitant about some things in a way other parents might not be because we realise life can be snatched away.

"It was brought home to us in 2007 when we took a holiday in Praia da Luz, Portugal. We were a few minutes away from the development where a few days after our departure Madeleine McCann was snatched.

"Events like that make you even further appreciate each and every day with your child and we do. Our blonde, blue-eyed bundle of joy certainly never suffers from a shortage of cuddles and kisses from her mum and dad.

"She's such a cool kid but although I adore her I'm also aware that she has a privileged life so I'm quite strict as see my job as making sure she turns into a happy, responsible adult."

Recently, competing as one of the celebrities challenged to reach the Arctic Circle, in ITV's 71 Degrees North, he surprisingly received a confirmation that his childhood behaviour and problems, following the tragedy, were understandable.

He had to be assessed medically and psychologically before being allowed to take part and says: "I've always known I have an inner strength and survival instinct and didn't think I'd been horrendously damaged by what happened to me as a kid.

"But the psychologist did remark that 'how I had been dealt with at the time was totally wrong'. Even though I'm now 46, and it's so long ago, that was all I needed to hear. I don't need to sit in an analysts chair for months."

Health advice from fellow contestant and rugby player Gavin Henson has also transformed his health.

"I've ignored my health all my life really, but Gavin took time explaining about diet and exercise and now I'm a changed man.

"I've given up drink, and run, cycle and climb mountains and in four months have lost more than a stone in weight. I've never felt better," he beams proudly.

And recently, following completion of the book, he's realised that Conor would be annoyed if he thought his life was only remembered for the way it ended.

"He had a little roguish, fun life where he laughed and smiled a lot. So to only remember the pain and not to celebrate him would be wrong," Diarmuid says.

He's resolved that the book's publication marks a personal turning point. "I have to get over myself and grow up a bit, and stop wallowing and recognise that aspect of life, as well as many others, is over.

"Now I have to get on and look forward to the rest of my life doing all the things I love and being with the people I care about."