“I’ve always had a great interest in those who live to be over 100,” O’Donoghue continues. “Last year I read an article about the oldest person in the world passing away.

A few days later the next oldest person in the world also died, and within another week the next person had popped his clogs, so that got me thinking, what if there was this person out there killing all these centenarians? And if so, why?”

Why indeed. Sister Agatha’s motivation is that she’s been given a week to live and she wants to die as the oldest person in the world; therefore she takes off on a journey to bump off all the people who unlucky enough to be older than her. The newspaper stories might have been a catalyst for the story, but its anti-heroine is rooted in a real nun from O’Donoghue’s childhood.

“My mother taught in a convent when I was growing up, so I was surrounded by nuns,” he explains. “There was one in particular, Sister May, and she was just as naughty and as bold as you like, and I thought she was fabulous. I wouldn’t like to say I based a serial killing nun on her, but she definitely influenced the character.”

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Domhnall O’Donoghue’s ‘Sister Agatha: The World’s Oldest Serial Killer’ is available on Amazon, Apple Books and in all good bookshops.

Before turning to novels, O’Donoghue penned several screenplays, and this influence is notable in Sister Agatha, which plays like a comedic road movie taking in three continents and a multitude of set pieces. “The book is very much like a film in terms of its pace, its backdrop and characters,” the author agrees.

“I would love to think it might move into that territory and there has been some interest expressed from producers. There are very few juicy roles for older women. It might be difficult to get it over the line in terms of funding, but I’d like to think we could do a very pro-female aging story.”

The actress O’Donoghue cast in his mind to play the part is Murder, She Wrote icon, 90 year-old Angela Lansbury. “She’s never won an Oscar,” he says, “so one of my little fantasies is that she’ll get it for playing Sister Agatha.”

In the meantime O’Donoghue continues with his day job on Ros na Rún. “It’s great to be gay and portraying a gay character in a fair, balanced and detailed light,” he says of that job.

“The Ros na Rún audience is an older demographic, with more traditional views about things like marriage and relationships, but knowing what it had already achieved with gay characters before I came along put my mind at rest at the beginning. They really present Pádraig in a fair way. It’s never about a struggle with sexuality, it’s always about the whole person.”

There’s also another novel in the offing, with an outline already written and O’Donoghue raring to go once he’s done the publicity for book one. More than happy to be double jobbing, he believes the nature of his first love, acting and his new love, writing fiction aren’t very far apart. “I feel I am a much better actor because of my writing and a better writer because of my acting,” he says.

Domhnall O’Donoghue’s ‘Sister Agatha: The World’s Oldest Serial Killer’ is available on Amazon, Apple Books and in all good bookshops.