When the funnyman presenter told his morning radio colleagues of the modern penny farthing cycle he randomly bought more than a decade ago, they came up with a plan to have him take on the ultimate challenge - to cycle it down one of Dublin's steepest and longest hills.
The result, they hope, will bring glee to those who watch one of Ireland's most-loved comedians see-saw his way to the finish line - but also raise badly needed funds for an Irish children's charity.
It was set in stone after PJ's colleagues in Radio Nova, Jim McCabe and Clint Drieberg, got wind of his unique cycle.
"I stupidly said in front of Jim and Clint that I had a penny farthing and before I had any power over it, the idea had run away on me," laughs the star of The Young Offenders on navigating the precariously narrow bike across several kilometres of Dublin streets.
The plan is to raise funding for The Jack & Jill Children's Foundation, which provides home nursing support services to hundreds of Irish children with profound needs and their families.
"I'm going from the top of Howth hill to Hill 16 on a penny farthing on October 1. It's not the hardest challenge in the world when it comes to fitness, but it will absolutely be hard on the auld bum for the few hours that I'm doing it.
"I still have no idea how I'm going to get down Howth hill. Very few people worry about how you're going to get down the hill - it's normally how to get up the hill! For me, it's completely the opposite.
"I rode it yesterday for the first time on the road and I realised I should have just done a 100km or 200km cycle on a regular bike, instead of doing a 17km cycle on this ridiculous machine.
"But that's what I said, and that's what I have to do. It's amazing how brilliant Clint and Jim are in coming up with planning creative ideas when it's actually me that has to do the task."
PJ purchased the bike after becoming intrigued by it in a shop years ago.
"It's been sitting in the shed for years. I don't even know why I got it. I remember asking if it was for sale and he said '50 quid' and I said. 'I'll buy that for 50 quid'. It's been sitting there for 10, maybe 12 years, it hadn't even been serviced.
"You'd want to see the face on the guy in [Dublin cycle service] Cyclopedia when I brought him up a penny farthing to be fixed. This is a modern twist on it. It has a kind of mountain-bike wheel. It's like what someone in the 1500s would have imagined a bike would look like in the future."
PJ brought the penny farthing out for an early test drive this week and is scratching his head at the prospect of managing it for 17km.
"It's a real wobbly thing to sit on. It's not a stable bike. I've been riding bikes my entire life - BMX, motorbikes, mountain bikes. Being on two wheels has kind of been my whole life. But I've never ridden one of these for any longer than 30 seconds.
"That's basically going to be my training - me not actually training at all, and sitting around worrying about it. I was wobbling all over the place and everybody that sees you just starts laughing at you. I wouldn't say it was a booming start to the challenge."
Still, as part of Jack & Jill's Up the Hill challenge, those who take on a hill for the charity will make a very real difference to many families.
They include Edel White, who credits her family - especially mum Patty - and her Jack & Jill nurse for their support in caring for son Harvey (2). Harvey was born with profound special needs including cerebral palsy and epilepsy.
"From the time we met his nurse, Sinéad, we just clicked," said Edel. "Sinéad is just an angel. I don't have the words to explain what Jack & Jill do for my family and for Harvey. She's so reassuring and the things she's made possible for me to do - go for a walk, meet the girls, go for dinner.
"Even clothes shopping for an hour - she's made all that possible and so much easier. I don't think they get recognition for all the work that they do for special children and families."
PJ is delighted to see his comedian friends return to stand-up after 18 long months. But for now he hasn't any plans to take to the stage and had pared back on live performance even before the pandemic struck. He intends to focus on his radio career.
"I'm in no rush to get back to stand-up. I've been off the stage for a couple of years now. Stand-up I always found really stressful.
"Now that I have the radio and I'm there every morning, it's harder again to make that step to get back up on the stage again. I feel like a person who wants to be on the radio now and that's the main focus."
To donate to PJ's challenge, or to register your own Up the Hill challenge, log on to jackandjill.ie/events/upthehill2021/, or via the Radio Nova website. Donors can text WECARE to 50300 for €4 and Jack & Jill will receive a minimum of €3.60 (provider: LikeCharity helpline 0766 805278).