You can rhyme about anything in hip-hop. Every topic under the sun has already come up for consideration in front of the microphone. From gold chains and Burger-King bathrooms to obsessive fans and height issues, there’s very little which hip-hop has not already put through the lyrical wringer.

But there's always room for something new. Until Macklemore and Ryan Lewis decided to turn their focus on the subject, very few hip-hop acts have focused on shopping at your local charity or thrift shop. Hip-hop likes its fashion, but it tends to concentrate on bragging about aspirational high-end brands rather than saluting the joys of low-cost, frugal second-hand gear and the clothes you can find on the racks in charity shops.

Thrift Shop bucked that convention – and it also changed life for the Seattle rapper born Ben Haggerty and his producer Lewis as well. While Macklemore and Lewis (the latter working with the rapper for the past number of years first as a photographer and then producer) had already a strong, vibrant underground following, nothing tickled the mainstream's fancy until Thrift Shop came along.

Even now, millions of sales and more millions of YouTube views on, it's still ubiquitous, a track which seemingly gets a new lease of life every few months. You can understand why it's hit a chord. It's a booming, thumping, tongue-in-cheek paean to charity shopping with an infectious chorus which digs its way deep into your brain and declares squatters' rights. The video is equally eye-catching, with Macklemore rocking a gigantic fur coat and a bunch of other thrift-store finds.

People can’t get enough of it – or the duo behind it. When they hit Dublin late 2011, they played two shows at the Twisted Pepper. In two years, this has become a brace of sold-out dates at the much larger O2 arena. That’s a hell of an acceleration.

This growth in profile and popularity is even a surprise for the man at the heart of it all.

"When we made Thrift Shop, I didn't think there was any chance that we would have a shot at commercial radio whatsoever, no chance. I thought if we didn't sign a major- label deal that there was a percentage of a chance that it would take off at radio.

"It's weird to be recognised in public now as the Thrift Shop guy. I didn't see that coming. I never thought it would get nearly this big, so I feel an immense sense of gratitude that people are connecting with the music as they are."

What’s just as hard to take is that it shows no sign of stopping.

“When the record took off, it just kept on going and it’s not showing any signs of slowing down. It’s exciting and scary at the same time. It’s like everything is largely the same as it was and I’m the same person I was before all this happened, but life still feels like it’s totally bizarre and strange and changed. That’s hard to adapt to.”

Rewind Macklemore's life and career and you can see how unlikely all of this must seem to him. He first started rapping as a 15-year-old and released his first tracks back in 2000 as Professor Macklemore. The spoken word, open-mike scene in Seattle was where he spent his apprenticeship – "reading a poem with no beat in front of 20 people", he has said before, "is way more difficult than playing to a huge crowd".

Slowly and surely, the white kid from the self-professed privileged background gained a foothold on the local scene and began to draw a loyal crowd when he played live. But what really pushed him to do more than just be a local hero came after a stint in rehab to get over drug and drink addictions.

He found himself living in his parents' basement and thinking that it was perhaps time to give up and find a real job. Enter The VS EP in 2009, the alliance with Lewis and Macklemore was fit for purpose again, though it did take a struggle to get there.

“I was close to giving up”, he says. “I’d always thought that if I could get and stay sober, I would be able to have a career making music. My drug and alcohol addiction was the one thing holding me back. I was over that and it was just a matter of writing the songs. I had to go for it, I had no choice.”

While Thrift Shop may be the track everyone knows from Macklemore, he's far from being an one-trick pony. There's also Same Love, Macklemore's anthem in support of same-sex marriages. It's a topic which is close to his heart, with two gay uncles and a gay godfather.

“Where I grew up, there were huge gay pride parades and my whole upbringing was around gay people. I wanted to write a song about gay rights and marriage equality for quite some time. But I didn’t know how to do it or what perspective to use. It was never about being the first rapper to write about this, but you also don’t want the song’s power to become diluted because it’s a bandwagon issue.”

Hip-hop is still viewed by many as awash with homophobic comments and anti-gay feelings. Macklemore knows that hip-hop still has some way to go when it comes to addressing gay issues or even accepting gay rappers, but he realises the importance of the message getting out into that world in the first place.

"The song carries a message that I want to be heard. It's a message about tolerance, equality, compassion and understanding. All artists want to have an impact on people. I know Same Love is not a song that's you listen to and perhaps immediately agree with everything that I say. Everyone interprets music and messages differently.

"But I hope people listen to Same Love and then have a conversation or re-evaluate the language they use. When the song works live, when people respond to the message, it's a beautiful thing."