Joseph O’Connor’s mammoth new novel, Redemption Falls, opens with a stark, striking image: a woman, young, alone, dishevelled, walking across Louisiana. As the pages turn, the reader cannot but be dazzled by the book’s vast cast of characters, enormous historical canvas and ambitious structure. Amid the chaos of the last gasp of the American civil war, the story unfolds courtesy of a fistful of narrators, a raucous succession of poems and ballads, and a roller-coaster series of literary forms, including transcriptions of recorded interviews, historical documents, letters, poster-bills, even paintings.

But there’s always the character of Eliza Duane Mooney to hold on to. Which is just as it should be because, according to O’Connor, she was the spark that created the entire book. “I got a picture of this woman walking through the landscape”, he says. “That’s often how a novel starts off for me. At first I wasn’t sure whether it was a contemporary novel; whether the woman might be Afghani, or African. Then I saw a connection between this character and the narrator of Star of the Sea. So I stuck with her.”

Star of the Sea, of course, is the novel that propelled O’Connor into the international best-seller lists three years ago and that, to date, has netted him some six literary awards, including European Novel of the Year. The new book isn’t exactly a sequel, but there is a connection in that the lone walker is the daughter of a couple who crossed the Atlantic on that benighted Famine ship. Like many Irish immigrants of the period she has embarked on a new kind of life on a new continent, hoping for peace and prosperity - only to find herself caught up in the horrors of someone else’s civil war.

”I wanted to write three novels about Irish-America, set 20 or 30 years apart”, O’Connor explains. Originally, he envisaged them bound together “like Roddy Doyle’s Barrytown trilogy”. He holds his hands about a foot apart; then, with a grin, moves them gradually outwards. Star of the Sea runs to some 432 paperback pages, and Redemption Falls weighs in at a hefty 457. “But I think we’ve passed that point now, unless the next one is a very slim volume indeed.”

Few of O’Connor’s fans would have predicted that super-sized historical novels would prove to be his literary genre of choice. Before he wrote Star of the Sea he was best known for sassy satire in the shape of The Secret World of the Irish Male, The Irish Male at Home and Abroad and The Last of the Irish Males. He had also flirted with other genres, including travel writing (Sweet Liberty: Travels in Irish America), thriller (The Salesman) and police procedural, part picaresque love story (Inishowen), as well as being the author of three screenplays and three stage plays. But, he says, the runaway success of Star of the Sea came completely out of the blue.

“When I started writing”, he says, “I used to get the trade magazine The Bookseller, and it would have figures for writers like John Grisham whose books sold, say, 12,000 copies a week. And I remember thinking that it couldn’t actually be true. No book would sell that many copies. Then I remember thinking that no book I would write was ever going to sell that many copies. So when it happened, it was dream-come-true territory. I sat at the computer looking at the figures. Star of the Sea started the day at number 360 or something like that, and just went up and up. Then it became clear that it was going to go to number one.” The novel became a runaway bestseller. “The whole experience has been brilliant fun, and I wish every writer could have it once in their writing life; and if it never happens to me again, I don’t mind.”

He didn’t, he insists, set out to repeat the trick with Redemption Falls. “I don’t think too much about the marketing and sales end of things, or even reviews and stuff. That’s not my job”, he says. “I just try to write the book I want to write - which is also the book I want to read - and not allow expectations to be part of the picture.” Having written one historical blockbuster, he was reluctant to begin another. But then he was awarded a fellowship at the New York Public Library’s Cullman Center. “It was amazing”, he says, “to go in to work in this wonderful building on 5th Avenue and 42nd Street on a Monday morning.

“And the best thing about it is the librarians, who are remarkable people. If you say to them, “I’d like to know about Irish emigrants in the American civil war between April 1861 and November 1862’, they know where to get the stuff - and they just bring it to your desk. Down in the basement they have the papers of a crowd called the Sanitary Commission, who ran the hospitals, and who were the first port of call for the families of soldiers who didn’t come back. They have something like 8,000 letters from Irish people in that collection, and each of those letters is a novel waiting to happen.”

O’Connor was also deeply moved by the library’s archive of interviews with former slaves. “In the late 1920s, somebody had the bright idea to record the testimonies of these people in their own words. It’s extraordinary that slavery lasted into the era of technology - that there are transcripts of people who were born in Africa, were slaves in America and were freed by the civil war.” O’Connor used the orthography of one of the documents to tell the story of another character, Elizabeth Longstreet, who’s employed as a cook by the central pair in the story, the Irish-American revolutionary Giacomo O’Keeffe, and his wife Lucia, who is a poet.

The most extraordinary character in the book is also, however, the strangest. Jeremiah “Jeddo” Mooney - Eliza’s younger brother - is a child soldier. Where did O’Connor get this most chilling of images? Does it have anything to do with the fact that he is himself the father of two boys, aged seven and three?

“Well, it came partly from historical fact”, he says. “In the American civil war, there were 100,000 child soldiers - which is defined as somebody under 16, but in some cases they were as young as seven or eight. And of course they’re such terrifying figures in our own world; kids whose childhood is over. What’s so poignant is not simply the image of a child with a gun, but the knowledge of the abuse that has been perpetrated on that child.”

His own writing has, he says, changed since he became a father.

“When I look back on my early books, I think I was really waiting to have kids - they’re all about parents and children and divorce and separation. It’s probably no secret that I had a turbulent childhood, so no doubt that’s in the mix somewhere. But for me it’s much more to do with having children, and having the experience that every parent has, of seeing the world through new eyes. Everything in my life has gone well for me since I had my kids. They’re like a lucky charm for me. I’ve had no bad luck since they were born, and I tell them this every day.”

Lucky kids. In a way, Jeddo too gets lucky - though at a pretty terrible cost.

“There is a kind of redemption in the book”, says O’Connor, who says he’s happy to describe himself as a socialist - and an optimistic one, at that.

“I hope it’s not sentimental, but I hope it says that while the world is full of hangmen and torturers, it’s also full of Eliza Mooneys and Lucia O’Keeffes. It’s not a big Hollywood ending, but there is just a sliver of hope in there.”

Meanwhile, O’Connor’s next book really will - he promises - be short and sweet.

“I have an idea for a novel about a woman like the Abbey actress Molly Allgood, who had a secret and passionate affair with JM Synge towards the end of his life. After he died, she moved on and had a major career on Broadway. So it’s also the story of how America dealt with Ireland in the end; by putting it on stage and sentimentalising it. I think that would be a good end to these Irish-American stories.”