The water temperature is around 9°C - far from freezing - but there's something about jumping straight in that feels like being hit in the chest with a lump hammer. I've been cold before but truthfully nothing like this.
This is the kind of cold that soaks right through to your bones in seconds. It's so cold, in fact, that getting out of the water and standing dripping on the stony beach feels positively warm. And yet, at 8.20am on a grey Thursday morning in December, there are around 30 people at the cove in Greystones, Wicklow, to partake of this daily ritual.
As recently as November, that number was apparently up to 60 a day or 100 on weekends. I eagerly accept a cup of steaming hot tea and a piece of gluten-free Madeira cake from David Flynn - one half of the Flynn twins better known as The Happy Pear - and he asks me, "Do you get it now?"
And I have to admit, I do. It's invigorating and I don't think I've ever felt so alive.
The other swimmers gathered around in Dryrobes and flip-flops are amazed that after 20 years of living in Greystones, this is the first time I've swum in the sea there. And while I've always meant to do it, it was an invitation from the Flynn brothers that got me here for a sunrise swim.
The occasion is a chance to talk to the Flynns about their new book, The Happy Health Plan, as well as the ins and outs of running a health-based business that employs 150 people and has made the pair social media stars.
"I think this is the seventh year we've been swimming here every day and the numbers have grown steadily in that time. First there was a small group of maybe five people, then that became 10 and then 20. This year the numbers exploded during lockdown because people didn't have much to do and they realised that a swim in the sea is healthy, gets you outdoors, doesn't cost anything and gets your day started on a fantastic adrenalised high," says David.
"We swim at sunrise, which is around 8.30am at the moment but is around 4.50am in the summer. It doesn't take long: just jump in and out and you reap the rewards all day. There's also something primal about watching the sun come up over the waves. You get this amazing light in the morning and you become plugged into the tides and what phase the moon is in."
Spend any time with the Flynn brothers and it quickly becomes apparent that what you see is what you get.
They are relentlessly positive people who describe themselves as naturally optimistic by disposition. But behind the social media presence, the handstands and topless six-pack selfies, and the consistent message that plant-based eating is good for you, there are also two canny business people and a business that's been going for well over a decade.
There are also busy personal lives. David was married to his partner Jan but has been separated for six years and together they have two daughters, Elsie (10) and Izzie (7). Today he's with a new partner, Sabrina.
Stephen is married to child psychologist Justyna and together they have three kids, May (10), Theo (7), and Ned (4). The twin's brother Mark also works in the company.
Throughout the interview we do upstairs at the cafe in Greystones, around the corner from the cove, the pair's kids wander through to grab a cuddle with their dads before school.
"The trick to cooking for kids is: don't ask. Cook what's good for them and put it down in front of them.
"Make sure they haven't had snacks between meals and they're much more likely to eat it.
"That said, while we're both vegans, the kids aren't. We're not hardcore about that at all," says David.
Stephen adds that his wife is Polish, and trips back to Poland were hard enough for himself as a vegan, but asking his mother-in-law to feed the kids vegan food was a non-starter.
"I really wanted our oldest to be vegan when she was born but then we went to Poland.
"Our family there live on a farm and have all their own animals and they wouldn't know what a vegetarian was if you explained it to them.
"Good luck trying to leave the kids there and telling them they can only eat veg. I quickly learned that it takes a village to raise a family,' he says.
When the kids are at home, they eat a vegan diet but when they're out, they can eat what they like.
"Our message has softened in recent years. We're not about pushing veganism or vegetarianism: they're binary terms and aren't helpful.
"There is no 'perfect' but we want people to feel good and have better health, and that means eating a predominantly plant-based diet," says Stephen.
"Being a vegan doesn't automatically make you healthy. You could eat a diet consisting of vegan sausages and vegan ice-cream and vegan doughnuts and still be incredibly unhealthy."
The Pear brothers want more people to turn away from processed foods that have been overly interfered with to increase shelf life, and to make them look and taste better, at the expense of their health.
"Processed food is designed to hijack your mammal brain. Our prehistoric ancestors had no access to refined sugar and refined fats: it was all about roots and shoots," says David.
Despite having published multiple vegan cookbooks in the past, The Happy Health Plan is the first health-based book that the brothers have released.
It contains 90 recipes designed specifically with medical experts to help with various aspects of health.
Under the microscope are heart health, skin condition, gut health and weight loss.
Significantly, the brothers say a key part of this initiative is that there is no calorie counting to be done and readers adopting the plans can eat as much as they want.
"This is the big one, as far as we're concerned. Over 50,000 people have been through the Happy Heart and Happy Gut courses we've run, and this book contains what we've learned in the process.
"We've built these courses, both in person and online, with the advice of doctors and dieticians, and they're medically sound," says Stephen.
"The book also has a lot of stories from people who have improved their health and who enjoy their lives more as a result. It's all meant to be highly relatable. All the recipes are extremely tasty, but it's the first book we've done where that hasn't actually been the main point - this is about health and it happens to be tasty. It's all low-calorie food, high in fibre and low in energy density."
The big message is that people aren't eating enough fibre and it's having an effect on their health.
So at this point, an awkward question has to be asked. How do they know? After all, the Happy Pear aren't doctors and don't have medical qualifications. What puts them in a position of being able to offer others advice?
"It's a good question. This is about hard data, not two lads in a vegetable shop selling veg.
"We started out reading books by people who really impressed us, like Dr Dean Ornish, who was able to reverse the indicators of cardiovascular heart disease, the biggest killer in the world, through diet.
"And we started realising that this could really help people so we read more," says David.
"Next we went around the corner here in Greystones to Dr Brendan Cuddihy and asked could we borrow a nurse to help us run an experiment.
"We followed Dr Ornish's advice with a group of 20 volunteers and put them on a diet designed to lower cholesterol and improve heart health, and we measured their cholesterol, weight and blood pressure before and after.
"And we ran it as a cooking course, basically encouraging people to eat vegetables.
"And we really didn't know what would happen but the results were amazing.
"There was a 20% drop in cholesterol; people lost weight and reduced their blood pressure.
"From there we started to offer this course online, and we travelled and presented talks on the eating and cooking end of applying this scientific research."
The Flynn brothers have given talks at the Royal College of Surgeons in Dublin and at medical conferences in the UK.
They ran a Happy Heart course for 75 medical professionals in the UK in one go.
"The funny thing is that we were really nervous going in and trying to teach these guys about vegetables.
"We had total imposter syndrome and I was sure we were going to be caught out. Who are we to teach these men and women anything? But what we got was a really warm and receptive audience," says Stephen.
"We gave one of our first presentations to doctors and when the first hand went up and the first question was asked, it was, 'Can I drink Diet Coke on this plan?' That was funny but it really taught us these are people too. They're normal human beings as well as being doctors and they have their own health concerns.
"It's one thing to know the data and a different thing to put it into practice in your own life," says Stephen.
Similarly, the brothers gave a talk at the London School of Economics at a medical conference for 300 doctors, and again were worried about their qualifications to be there.
The organiser took them aside and said basically, "You've helped 50,000 people through your courses and that's just as big an effect on people's health as any doctors in the room. You're just as entitled as anyone else to write a book on it."
In addition to reaching thousands via their courses, the brothers own some of Ireland's most followed social media accounts.
Their Instagram account has 550,000 followers, and an average post attracts 10,000 likes and hundreds of comments. Great for advertising a brand, not so great if you're of a sensitive disposition - people can be cruel online.
"Negativity can get to you. You can post something and have it be misunderstood or whatever, and just find yourself thinking, 'I don't want to do this anymore.'
"Overall, social is a double-edged sword. We're on a social mission so the reality is that if there was no social media, we'd be standing by the road holding up signs, so it allows us to reach people, and that's great," says Stephen.
"But at the same time... it's a beast that's hungry and demands feeding.
"People are sometimes envious of the size of our following but they should know that it comes at a serious cost.
"There are algorithms that need to be fed and you're playing a game in which you're feeding the beast. And of the 500 comments left on a post, there might only be two or three that are negative but they're the ones that stick with you."
'The Happy Health Plan' by David and Stephen Flynn, Penguin Life, (£16.99)