The Derry man says black humour was the backbone of his family's survival when she died of cancer and his dad reared 11 kids on his own.
Séamas has told the story in his first book, Did Ye Hear Mammy Died, which has gone to the top of the Irish Times bestseller list.
The dad-of-one, who writes a parenting column for the Observer, is better known for cracking jokes about the tribulations of raising a toddler.
But he's not afraid to look into the dark corners of his grief after losing his mother Sheila when he was five and she was just 43.
His father Joe reared their 11 children, aged two to 17 after she died, and the writer says humour, as well as their unbreakable bond, helped get them through their loss.
"It's an Irish thing. Northern Ireland in particular has a strand of black humour, and there's probably a reason why people in Northern Ireland got used to laughing at things that were sad and tragic and horrible.
"In our case in a family environment that's normal," he says. "You see dramas on TV and think was there no one here making a joke.
"That's how it's always been for me, a funeral where people are laughing their heads off or a wake where someone is singing."
The title of the book came from the question Séamas asked people at his mother's wake as a bewildered five-year-old, and writing it gave him the chance to pay tribute to his father.
The retired civil engineer, now 74, has given his blessing to the book along with the writer's 10 siblings.
"He has the work ethic of a soldier ant but is dazzlingly addicted to trash culture because of daytime TV. He grew up in the Sixties but sometimes it feels like he was dropped on to the planet from Venus," says Séamas, who lives in London with his wife Ciara.
"We flatter ourselves that we appreciated our dad. We climbed him like a tree when he came home from work and we hero-worshipped him.
"We didn't have much money and my dad was on a single wage, working nine to five and doing all the things like driving us to football matches, orchestra, choirs.
"I would love to think on my best day I could manage a week with three or four kids."
Séamas has been touched by messages from people who have been moved by his family's story, or who knew his mother.
It's allowed the 35-year-old to collect little bits of information about her and clear up some of the confusion in his own recollections of her death.
He clearly remembered that Bryan Adams was at number one for 18 weeks in 1991 with Everything I Do, but not that his mother had died during that time.
She'd worked as an Irish teacher before her death from breast cancer, after years of battling the disease.
While writing the book he read letters she'd written to friends about her illness which brought home to him what his mother had endured.
"I have siblings now who are older than she was when she died, and they are no age. It just makes you so sad," he says.
"Writing the book put me in her position for the first time. I got to read her letters about her diagnosis and relapses, the cancer came and went.
"I'm reading her account of this and it made me feel ashamed that I had never realised what it was like for her. She knew she was dying, and she had 11 kids.
"That was the biggest task, making sure I didn't shy away from the stuff that was really hard."
One of the other challenges of writing about his family life was how to include everyone.
In the end he decided to leave a lot of them out.
"In every room where things are happening there are 10 or 20 people there," he explains. "The book is mostly about me and my self-absorption as a five-year-old, so my family are almost in the background."
His story stops at 11 and Séamas is already working on a fiction project. After a series of office jobs, he now writes for the Irish Times, New Statesman and New York Times but it was a Twitter story about drugs and a former Irish president which helped land him a publishing deal.
He'd previously been speaking to a publisher about a book and had his family's incredible story in mind, but it didn't come to anything.
He later shared a memory about working as a waiter in a Dublin music venue at 18 when he thought he'd got the day off and took some ketamine to relax, and when the story went viral several publishers approached him about a book.
"I was relaxing with some party-fuelled powder when someone rang and told me to get into work to do a VIP service.
"Mary McAleese was being shown round the venue, and by 4pm I'm slowly disassociating from reality and my brain is melting.
"It went viral on Twitter and four or five people came forward and said you should write a book, and I had this idea ready.
"But writing the first chapter was the hardest thing," he says.
Did Ye Hear Mammy Died by Séamas O'Reilly, published by Fleet is out now.