It happened 41 years ago but I still recall locating the gate into a field near Turlough village in Mayo, where the caravan of 73-year-old Sheila Fitzgerald - whom I had hitch-hiked from Dublin to meet - was parked. It was 1977 and I was 18. The field seemed dark until I saw a faint glow emanating through her curtained windows. My hands blindly made contact with a caravan door. I knocked and it opened in a luminous square of light. A tiny, radiant woman stood there with white hair and a delighted smile. “You found us,” Sheila said. “Isn’t life exciting?”

It felt like glimpsing a flame of energy as I entered her den, crammed with books and heated by a wood-burning stove. Three cats bestowed hospitable gazes upon me. Sheila’s address was The Ark, Turlough, Co Mayo, and her caravan was an ark of happiness and refuge. Yet even its name was linked to one of the many tragedies in Sheila’s life - having been named by her teenage granddaughter who had died tragically in Kenya two years previously.

Our friendship started in 1977 when Sheila read something about me. She sent me a postcard saying, if ever in Mayo, I was welcome to sleep in her caravan and use a nearby wood as a sanctuary in which to write poems. There was a 55-year age gap but, because I had lost my mother and Sheila had lost a son, we felt an affinity. Sheila opened my eyes to new ways of seeing things. Although penniless, she was the richest person I knew because she wanted nothing. She had her beloved dog, Johnny, her books and the cats who shared her caravan, along with numerous callers of all ages, inspired by her positivity.

Sheila would stand out in any generation. She removed the hands from the only clock in the Ark and wrote the word ’NOW’ across the clockface. This clock always told the right time, because Sheila lived in the present. This did not mean the past was forgotten. Her walls were lined with pictures of family members she had lost. I became a regular visitor. If you dream of being a writer, you need one person to take you seriously. Unemployed in Dublin, I’d hitch-hike to sit in her candle-lit caravan and read her my latest poems. We always talked long into the night and she was unafraid to address her tragedies. The stories about her in my new novel were stories I heard on my first visit to her Ark.

Sheila had the gift of being true to herself and passionate about causes she believed in. The artist Pauline Bewick remembers her as a tiny crusader, covered in flour hurled at her by an outraged citizen when Sheila took part in a protest. As a child, the poet Paul Durcan attended her innovative art classes, sparking his life-long passion for painting.

Sheila’s tragedies might have overwhelmed a lesser soul, but at her lowest ebb - having lost her son, daughter and only granddaughter - she made a deliberate choice in old age to embrace happiness. Her happiness came from refusing to be conquered by grief. She was happy not because of life, but in spite of what it stole from her. At 18, I was too inexperienced to understand how much easier it would have been to sink beneath the weight of grief. But in the years since, when bad things have happened, it is Sheila I try to emulate when picking myself up, borne forward by the inner strength of this woman who first told me her life story four decades ago.

Born in 1903, Sheila was one of five Goold-Verschoyle children raised in Dunkineely, Co Donegal, amid a babble of debate where no viewpoint was taboo. Her brother Neil inherited the family property as eldest son, but Neil Goold (he dropped the Verschoyle surname) rejected his inheritance and become a communist. He worked in Dublin’s worst slums and was jailed for communist agitation. Brendan Behan’s mother, who considered Neil a saint, sheltered him in her Crumlin home before he moved to Moscow.

Sheila’s beloved youngest brother volunteered to fight with the Soviets in the Spanish Civil War. Growing disillusioned, he was tricked onto a Soviet ship and disappeared, dying in a Soviet gulag, with their mother spending her final years desperately seeking news of his fate.

Sketching was Sheila’s childhood passion. Her father, a pacifist who supported Home Rule, was a utopian barrister who often defended locals without seeking payment. But the outside world intruded on Sheila’s paradise. She stopped sketching when two young men arrived who “brought the more complex world into our small oasis… [and] heralded the time to grow up”. The two young men included the man she married.

Sheila often talked about writing a memoir. But I never realised how seriously she longed to be a writer until I found her 1968 passport, listing her occupation as “writer”. A tattered envelope listed stories she wrote and comments from editors who’d rejected them. She was a separated woman of 65 when applying for that passport, travelling cheaply in Spain and Morocco, engaging with new ideas and people.

By 1992, when Sheila was 89 and living in a field near Kilmore in Co Wexford - where locals helped her to lead an independent life for as long as possible - it seemed unlikely she would write her memoirs.

We discussed the idea of my writing a novel about her life. For several nights we sat up in her caravan, making recordings about her life.

A year after her death, I found those tapes. They let me reconnect with Sheila when she was still in good health and see the totality of her life. I needed to write as a novelist and not a biographer, guided by a line from Sheila on the tapes about how she admired artists with the courage to shape reality into something new.

In 2005, I published a novel about her early life, The Family on Paradise Pier. But I’ve waited 13 years to publish An Ark of Light: this stand-alone novel telling Sheila’s later story, when she needed to make a new life as a separated wife in a society that didn’t recognise divorce, and as the mother of a gay man at a time when danger lurked everywhere for lesbians and gays. It explores how she found the courage to leave an unhappy marriage and start on a quest both physical and spiritual: to strip away the veneer of complexity and strive - despite setbacks - to grasp the joy at the core of life.

Eighteen years after her death and 41 years after she first told me her story, An Ark of Light is my account of that remarkable life. Sheila died in a nursing home in 2000. Her Wexford friends tried to honour her last requests. One friend made the small wooden box she wanted as a coffin. To reflect her radiance, it was painted in vibrant colours. Sheila wished to avoid the ostentatiousness of a black hearse, so her body was transported to Glasnevin Crematorium in an unassuming white van, before the body she’d outgrown entered the flames to the joyous finale of Beethoven’s ninth symphony. Sheila’s handmade coffin looked like a small boat that would cause only the barest ripple. But her influence is still rippling out to touch distant shores. I hope I have captured something of her uniqueness.