Don't know about you, but hearthrobs were different in my day. Leonardo DiCaprio had pinchable good looks, Will Smith was the funniest man in America and Robbie Williams practically wrote the book on bad boy charm (such a book would have later been recalled, for obvious reasons). Surely I’m not the only one who finds it curious that, a certain silky-haired Canadian aside, diminutive folk rapper Ed Sheeran has become the teen idol du jour?

Unashamedly scruffy with a mop of wild ginger hair, he’s far from Hollywood’s idea of the boy next door, but still, I’ve witnessed young girls nearly toss their cookies when he appears on stage. It helps that he’s a sensitive singer-songwriter type, but then, until recently, I didn’t realise that hormonal schoolgirls were into raps about weed and songs about prostitutes. This is the part of the Ed Sheeran puzzle that has people confused. One minute he’s a self-styled urban poet (see lyrics like: “I’m up and coming like I’m fucking in an elevator”); the next, he’s a starry-eyed acoustic softie, and for the life of me, I can’t figure out which of these personae has the girls all aflutter. Unless it's both...

I meet Sheeran in the penthouse of the Morrison, where he’s biding his time before an appearance on The Late Late Show, aka his gran’s favourite programme. Not a single fan is lurking outside the hotel, so either he’s not quite at police barricade level yet, or his trip to Dublin is a very well-kept secret.

“I haven’t got to the stage where I can’t walk down the street,” he grins, “but I have got to the stage where I have bright ginger hair and a few songs in the charts so I generally don’t walk down the street!”

Already, Sheeran is coming off as a rather modest chap, but there was no masking his popularity last month when he performed to a heaving Olympia crowd as part of the Guinness Arthur’s Day celebrations. Unless, of course, all those screams were coming from his Irish relatives…

“If all of my family came to the Olympia show, we’d be the Olympia show,” he laughs. “I just had my mum and around six cousins, but that’s hardly anything!

“But I did two other small shows. I played the Ruby Sessions with my cousin Laura (also Sheeran, a maker of wonderfully dark electronica – Ed) and then we did a secret one in another pub. That one was a bit weird. Nobody gave a fuck. They were just there with a Guinness, chilling out.”

It’s hard to believe it now, but Sheeran is no stranger to bad pub gigs. In fact, the Halifax lad’s success has been anything but overnight. It took 1,000 gigs, five self-released EPs and four years of couch-surfing before the major labels started paying attention to the lovable troubadour. Of course, once he signed with Atlantic records, the Ed Sheeran juggernaut was unstoppable. He’s now sold a million singles, has 27 million views on YouTube and over half a million followers on Twitter (to put it in perspective, that’s more than Joan Rivers, Liam Gallagher or Ricky Gervais). Despite being only 20 years old, he’s swiftly closing in on what he describes as “Coldplay-sized fame.”

“I’m not gonna lie, that’s always been my plan,” he says, “but I didn’t think I would have done it so soon! But I don’t think you should get into being a singer-songwriter if you don’t want to do the gigs. Obviously it’s therapy and it’s a way to get your songs out there, but for me I wanted to get my songs out and play them to people, play big gigs! That’s what I wanted from the start.”

Yesterday saw the premiere of the fame-centric music video for new single ‘Lego House’, starring fellow ginger heartthrob Rupert Grint as Sheeran. It’s a truly brilliant video with a truly brilliant twist: who’s clever idea was that, then?

“Erm, mine…” he says, bashfully fiddling with his shoelaces. “I tried for ages to get Grint in the video. Ever since the Harry Potter films have been coming out, I’ve had the Ron Weasley thing, like from day one of being at school! I thought it would just be fucking wicked to take the piss out of myself and get him to play me in a video and do all the things I do, like go on stage and write songs. And he was wicked, he was just the perfect guy to do it!”

Alright, this calls for some highly intensive investigative journalism. I’ve seen the man break into 50 Cent’s ‘In Da Club’ on stage, but hanging out with Ron Weasley is hardly doing much for his street cred. Is Sheeran a bad boy or isn’t he? To begin with, I’ll need to know the reason behind this Lego obsession of his. I’ve seen it on his artwork, on his Twitter page and, now in the new video; what’s the deal?

“I like it.”

Er, that’s all well and good, Ed, but spending your nights building a tiny tugboat for a little plastic Jack Sparrow isn’t very rock ‘n’ roll, is it?

“Isn’t it? Or is it really rock ‘n’ roll?”

Moving on. Any tattoos?

“I’ve got four. I’ve got a plus, a paw, a jigsaw piece and a little star.”

Nothing seedy or untoward there. +, of course, is the title of Sheeran’s debut album, now inked onto his wrist for all eternity.

“It fucking hurt as well,” he smiles, “it was bearable, but it really hurt.”

What about stage-diving?

“Oh, yeah,” he says, sheepishly, recalling a recent show in Leeds. “I stage-dived for the first time and I got dragged out by the bouncers and then my tour manager was like (adopts headmasterish tone) ‘Don’t do it again!’ Then about 20 seconds later I jumped back in the crowd! That was rock ‘n’ roll. But I’ve done more rock ‘n’ roll things than that! About a month ago I stayed ‘round my mates’ flat in Camden and I used his curtain as a duvet. That’s alright, isn’t it?”

Next category; celebrity mates. Now, I know he’ll do well here. He’s already had big-ups from Elton John and Jamie Foxx, the latter having invited Sheeran to sleep on his couch, long before he became a household name.

“I went to Beyoncé’s album launch,” he offers. “That was sick! My mate was like, ‘I’ve got a pass for the launch if you want to come?’ so I was like, ‘Awh, safe!’ It was amazing. I managed to blag myself into the little VIP box that they were all in: Gwyneth Paltrow, Jessie J, Adele, Tinie Tempah, Jay-Z. I mean, compared to the people in this box, I’m a nobody!”

At this point, I can only hope that he’s got a background in shoot-’em-up gangsta rap. No such luck. With Sheeran’s parents hailing from Wexford, it’s clear that Irish music played a big part in shaping his sound.

“I was brought up on Van Morrison’s Irish Heartbeat album,” he tells me. “Then there was Planxty, Christy Moore, Donal Lunny and my cousin got me into Bell X1, The Frames, Damien Rice, Mundy and Damien Dempsey. I’m really inspired by Irish music.”

A traditional Irish folk song, ‘The Parting Glass’, even made it onto Sheeran’s record and for my money, it’s the album’s loveliest moment. Any reason why he squirreled it away as a secret track?

“13 tracks is unlucky! I was not gonna put out an album with 13 tracks!” He catches me raising an eyebrow. “I’m not superstitious! But if the album flopped, I’d be like, ‘That’s why!’”

But the album didn’t flop. Not even close. Sheeran clearly hit on a dynamite formula with +, and one that could probably keep him in Lego sets for another half a dozen albums.

Somehow, I don’t think he sees it that way.

“The key to failure is trying to please everyone,” he warns. “If I make a really obscure album but I think it’s the fucking tits, I’m gonna push it and if the fans don’t like then I’m not gonna be upset. But if I make an album that I know that the fans are gonna lap up but I don’t really like it, it’s just gonna be a bit pointless. Why would I do that?”

Half an hour later, and I’m no closer to solving the Ed Sheeran puzzle. My best guess is that he falls somewhere near the halfway point between hip hop bad boy and sensitive good guy. I do know this much, though. Wherever he sits on the rock ‘n’ roll spectrum, right now the view’s got to be pretty damn sweet.