I am approaching Marina Guinness's house in Celbridge one sunny autumnal afternoon, marvelling at how few potholes there are in the drive when I am accosted by a most incongruous apparition.
A perfectly angelic young man with a shock of platinum blond hair is hoeing ragwort out of the gravel while wearing a pinstripe suit.
As I park the car I spot Glen Hansard whizzing past on a motorbike. It is not unusual to bump into famous people at Marina's - Grace Jones in the garden, Jerry Hall in the kitchen, Kodaline in the ballroom, but this creature stands out even here.
He stops hoeing and stretches out a hand to greet me and I am dazzled by his smile and the warmth of his vibe.
"This is Oscar," Marina says. "And this is Scarlett. They're playing at the picnic. We're so excited to have them!"
Scarlett, with long, dark hair and wearing a glorious satin evening dress steps forward and grabs my hand with equal enthusiasm.
The picnic is the Electric Picnic and it transpires that the couple are in a band called Khartoum, with their friends Jake and Cam. Scarlett is the drummer and Oscar is the singer.
I panic, momentarily worried that I might have to hoe weeds too, but Marina drops her gloves and announces that we are off on an excursion to her cousin's steam museum in Straffan. "They have a tea room," she says. "With Protestant cakes. Almost pleasure free."
We pile into the Nissan Micra and head off.
Oscar is the grandson of Marianne Faithfull, an old friend of Marina and my husband Shane.
He loves Ireland. "I know Marianne is not Irish," he tells me. "But I think of her as my Irish granny."
It's hard to picture Marianne, the singer and actor, as anyone's granny. In my mind she is eternally at the epicentre of the Swinging Sixties, when she was known as an iconic fragile beauty, with huge, innocent eyes hiding behind a heavy blonde fringe, sensual pouting lips and an aura of primal sexuality.
She became famous when she was 18 with a Rolling Stones song called 'As Tears Go By, married John Dunbar (who owned the gallery where John Lennon met Yoko Ono) had a baby (Nicholas Dunbar, Oscar's dad), left him and shacked up with Mick Jagger and was catapulted into tabloid hell after a drugs bust at Keith Richards's house where she was discovered wearing nothing but a fur rug.
Her life spiralled into heroin abuse, homelessness and suicide attempts, and she lost custody of Nicholas.
If that wasn't enough, her great great uncle was Baron von Sacher-Masoch who invented masochism.
I don't even know if Oscar can sing, but if he can, with all that DNA I feel he is destined for greatness. Then I picture him in rehab, and I want to tell him to stick to the weeding.
At the tearoom, he insists on paying for the cake, which he actually eats, adding to the charm.
On the way back in the car, we talk about pop stars being divas. Marina asks if he has read a savage piece Lynn Barber wrote about his granny. He hasn't even heard of Lynn Barber but finds it on his phone and she persuades him to read it out loud to us.
Lynn is typically bitchy about how Marianne ignores her presence while "trussed like a chicken in Vivienne Westwood with her boobs hanging out", and it's sort of funny, but when she gets to describing her "angry red mottled arms" from being a heroin addict Oscar chokes up and says it is making him emotional.
He doesn't like people being mean about his granny.
I mention that I write for a newspaper, and offer to do an interview, but I confess that the Marianne connection is a really good angle. "You don't have to be tortured to be a rock star," I say. "But it helps."
We agree to meet the following day at the Clarence Hotel and just before I leave he shows me the marks on the wall in the kitchen which measure his height from toddlerhood.
I pray that my contribution to his fame is not the devil's work.
For the photo shoot, he turns up in a shocking pink suit belonging to Scarlett, which as far as I am concerned seals the deal. Whether he likes it or not, this kid is going places.
Khartoum? I ask them. That's a weird name. As in the battle?
"Khartoum is the name of the horse in The Godfather," he says.
"Oscar is obsessed with The Godfather," Scarlett explains.
The band have been together for two years, Oscar met Scarlett on a haystack in a field in Shropshire, at a party that he was taken to by his grandfather.
"He was wearing my friend's dress and he didn't know anyone but by the end of the night all my friends had added him on Facebook for being such a hilarious little prince." Scarlett says.
The duo are perfectly complementary, she dark-eyed, buxom with a raucously earthy candour, him blond, ethereal and enigmatic.
"I auditioned and joined the band," she goes on. We got to know each other really, really, really well!
Before the band, Oscar went to drama school in New York.
"I got kicked out at 19, that was a heartbreak moment," he says. "But actually I got more work in television than anyone in my class."
He has acted in several TV shows including Casualty, and also appears in Absolutely Fabulous: The Movie.
"It was the disappointment of that which got me into music. My dad bought me a guitar, I had never played before. It was weird, I didn't learn chords and I didn't learn other people's music, I just started making sounds. I have always found music to be a great antidote to a broken heart, or a tough time."
He was also influenced by the American music producer Hal Willner, who has worked with Nick Cave, Lou Reed, and Johnny Depp.
"I love that man. My mum and dad fell in love listening to one of his records. I used to sneak off to see Hal where he was working at Saturday Night Live and I thought it was the coolest thing ever."
Jake, the bass player who is classically trained, tells me he can't get over the way Oscar manages to play guitar and write songs so well without any kind of lessons.
The band are deliberately visually striking. They made their debut at a fashion show in Paris - their videos and Instagram feed feature wild and imaginative outfits.
"We do love a bit of fashion," Oscar says.
I ask if he is ambitious, if he wants to be a big star. He laughs.
"I would just love to keep doing it, be able to pay my rent and have a nice dinner once a week," he says.
"You are hungry!" Scarlett shouts. "You want to get the most out of life!"
She turns to me.
"He is very ambitious, he is working all the time and he never switches off. And he looks after himself, he is not messing around."
I have to ask about Marianne's influence, I say.
"She is obviously an influence," he says.
"He has definitely caught the diva gene!" Scarlett giggles.
"She is obviously very inspiring, and she has amazing stories," he continues, seriously.
Do they put you off? I ask.
"It's a mix, there are some good ones and some bad ones…"
Are you planning to get arrested in a fur rug anytime soon? I inquire.
He laughs, and you can see a glimpse of the showman for a second.
"Of course! Can we do it here at the Clarence Hotel?"
But the showman is clearly kept in check by a desire to be more considered. "As a kid, my mum was more of an influence," he says.
His mother, Carole Jahme, is an award-winning writer and broadcaster, and he clearly adores her. But she and his father Nicholas went through a messy divorce.
"She was incredible, she was a single mum after my parents got divorced, and she was a disco queen. I remember being six years old and looking up at her spinning around the kitchen by herself, shaking off the stress, and I found that very inspiring."
I ask if having a rock star relation is a help or a hindrance.
"I think there is a misconception that being the grandson of someone like Marianne, you might have everything sorted for you, but it's not true. It's been a struggle. She is a support, for sure but we have got these gigs ourselves. Obviously she introduces you to amazing people, but if you don't have the goods, you are not going to get anywhere."
I still haven't seen them play live, I still don't know if they've got the goods.
When I get down to the Electric Picnic, they have played a successful slot on a big stage and they are exuberant, but things don't look promising for the next show. It's the afternoon slot at the same time as Christine and the Queens, there are very few people in the area. I worry for them when just as they are supposed to start playing, they all walk off the stage and the woman next to me asks if they have stormed off in a huff, and I am not sure. Maybe they do have the diva gene?
But they storm back on again and my fears are immediately dispelled. In fact my jaw drops. They are electrifying in every way. Oscar, in his pinstripe suit grabs the microphone like he was born to it and without a moment's hesitation he launches into song with a rich, resonant voice and an impressive range.
The songs are pure singalong pop, but manage at the same time to be grungy and primal, with intelligent, interesting lyrics. The woman next to me immediately starts dancing. Scarlett, still in her evening dress is a powerful, masterful presence on the drums and the three guys on guitar sing harmonies and balance out Oscar by being moody and motionless.
With stagecraft to rival Mick Jagger, Elvis and Beyonce, Oscar struts and preens and takes off his shirt and leaps into the crowd and dances in the mosh pit and jumps up on the speakers, never missing a beat.
By the time they finish, everyone is dancing, including an old lady, some hot young guys and a model in a leather trench and a Chanel backpack. They are so good that I am in shock, I am even shy to go backstage and congratulate them, but when I do they are sweetly grateful and tell me I am really nice to come and see them.
Later, film director Julien Temple tells me that Mick Jagger has been spotted dancing at their shows in London, a fact they opted not to disclose to me.
We talk about their future.
"It always feels like Oscar is the star of his own movie," Scarlett says.
"He paints a picture of what he wants and he makes it happen. He is not always sensible but he is a doer, and he dreams big."
You definitely feel things will happen for them. And you really hope they stay just as sweet as they are.
Khartoum play London's Screen on the Green November 15 @khartoumband