With more than 1,000 events under her belt, the celebrity wedding photographer has seen it all.
From dealing with toppling cakes and dresses catching fire to drunken groomsmen and tears of heartbreak, McCarthy has developed an unusual gift - she can predict if a couple will last based solely on their body language.
McCarthy said: "It has happened a handful of times. Weddings have been called off, or the couple would ring weeks later to say that they have split and won't need their photographs. And I have never been surprised.
"It's nothing to do with sniping - people can snipe and still be very much in love. It's more the energy between the couple. You can feel the tension.
"Sometimes the girl talks a lot to overcompensate for the guy, who is saying nothing. But there is a difference between the guys who aren't talking because they are shy and those who seem like they are just not interested in being there. There is usually just a bad vibe between them. It is a gut instinct."
She added her own husband Martin King says she "should be a psychic".
McCarthy has witnessed an increase of 40pc on the spend of weddings since the end of the recession and warned that couples should beware of taking on unnecessary costs.
She said: "During the Celtic Tiger, people tried to outdo each other - and I can see that starting again.
"At a wedding last year, every bridesmaid received jewellery from Appleby. At another, every bridesmaid received a designer pair of shoes. But most people waste money on needless costs such as wedding favours, an excess of flowers and paraphernalia.
"I see hotel staff sweep up 70pc of the wedding favours left behind at the end of the night. It's okay if it is a little chocolate that people can have with their tea - but I'm talking about pots of jam and a pair of dice with the couple's names on it. Who wants dice with other people's names on them?
"If you insist on favours, it is nice to place a simple card to say that a couple has donated to a charity on the guests' behalf."
On flowers, she advised: "It's okay if you are a high-profile footballer or celebrity and you can afford an extravagant spend - but then you find the average person trying to emulate that because they want their wedding to be better than everyone else's. Save it for a really good meal, a good band and the photos."
Meanwhile, recalling moments of joy and sadness that moved guests to tears, she said: "At one wedding, it was a bride's dream to have fireworks but it wasn't in their budget. The bride accepted it and moved on but the groom did everything to find a deal he could afford.
"He then convinced the bride to have the song Firework as their first dance to make up for it and, as the music started, the fireworks suddenly went off over her head. Seeing her face light up, while he stood beside her as proud as punch, it was just brilliant. That's what my job is all about. I will never lose that feeling I get from capturing a girl at her happiest and most beautiful on the day."
On some of the sadder moments, Jenny added: "I have had to take photos at a hospital or hospice before the service because a family member was too sick to make it. At another, a mother who had passed away had already left gifts and notes for her daughter's wedding. That was very sad."
Stressing the need for couples to remember the important things, she added: "I had a phone call after another wedding to tell me the mother of the bride had a heart attack since the wedding, before the photos even came back."
On the lighter side, Jenny revealed she will never work with bridezillas.
She said, laughing: "One or two have managed to slip through the net.
"I've seen brides get worked up because a special glass they had ordered for dessert wasn't available, or another demand their father-in-law go around the hotel puffing up all the pillows in the reception before guests arrive.
"Another bride, the way she spoke to her mother... it was disgraceful. If you're not happy on the inside, nothing will make a difference."
Jenny will host Weddings by Jen - a series of wedding masterclasses - at the Killashee Hotel in Naas, Co Kildare, on November 26 and 27. She will discuss everything from how to avoid the most common last-minute stresses, to tips on how to how to achieve the perfect look.