While selling bloodstock to help pay his way through a PhD, he met the woman that would change his life - both personally and professionally - in 2010.
At the time, Lucy Wiegersma was getting over the heartbreak of missing out on the 2008 Olympics in Beijing - an injury to her horse meant she had to forgo her place on the British team.
"Lucy came to see one of the horses I had for sale," remembers McCarthy, who had always been a showjumper.
"She quite liked the horse and she wanted to come back and see more and then she asked me out for dinner. That's how I ended up here."
The 38-year-old Tipperary native is now a long way from home. 'Here' is a farm in rural Devon where the Wiegersmas breed and train some of the best three-day event horses in the land.
A former showjumper in his youth, McCarthy is now eyeing a place in the Irish three-day eventing team for Rio 2016.
"I guess he wouldn't have got involved in the sport if he hadn't have met me, but he's got a huge amount of natural talent and determination," says his now wife Wiegersma.
"From when I set him off at his first event, it's all been his will that he's got this far."
As well as missing out on Beijing in 2008, the 2006 national champion also had heartbreak four years ago in London when she was first reserve for the British side which won a team silver medal.
"I'll get there one day," she tells BBC Sport about her Olympic prospects.
But having had a baby in April 2015, Lucy has taken a back seat as her husband has come to the fore, having seen him rise from 101 to 65 in the world rankings.
It leads to a fascinating dynamic - the man who has fallen into the sport could go the the Olympics before his wife who has worked on getting there since she was a girl.
"I'm very new to the sport and it's very exciting. I'm getting the opportunity to do things that I wouldn't possibly have got in showjumping," he says.
"She's delighted to see me come in and do so well in a short time, I think she gets a big kick out of that."
But Lucy says she can see a time where they could be competing against one another.
"Our careers have kind of crossed at the right moment," she explains.
"I've been taking a natural step backwards over the last couple of years and he's only really come into his ascendancy in the last year, so we've not really been competing against each other at the top level, yet.
"But I'm hoping to come back to it, so we'll have that joy to come in future years I hope.
"He's a fascinating person to watch on a horse. He has a huge amount of technical skill from years and years of showjumping, but he's not had so much experience cross country.
"He's very natural at cross country, but unlike most people of his age, he's not had all the knocks and the bangs that most people his age and that long in the sport have had.
"He rides with the confidence and the youthfulness of a teenager, but with the skill set of a much older person."
So does McCarthy think he would have found a path towards an Olympic chance without that meeting over a horse?
"No, quite simply no. Lucy's done everything, she's given me the opportunity to ride," he says.
"Without Lucy and her experience and her knowledge it wouldn't have been possible."
Whether McCarthy makes the Irish team or not will be decided later this month.
But even if he stays on this side of the Atlantic, a chance meeting over an equine sale has seen him add a wife, a child, and a new sporting talent to his life.
Oh, and in case you are wondering, Lucy did buy the horse.
They say that your whole life can turn on one chance event or meeting, and nothing could be truer for Irish rider Padraig McCarthy.