She herself had been shunned for speaking in her native tongue, having come from the generation who had no confidence in their Gaelic identity, and for whom speaking the language at school could get you caned. When Matheson's mother came from Barra to the mainland at 14 to work in a hotel with other island girls, they were forbidden to speak it. Using their own first language would have been viewed as slovenly.
There is, therefore, an astonishing irony to the life of Matheson, the auburn-haired, blue-eyed Celtic beauty who, through serendipity and sheer talent, has become one of the world's most famous Gaelic vocalists, as the lead singer of Capercaillie. Matheson is acutely aware of this, as an ambassador for the language that helped propel Capercaillie - the folk band founded by her husband, Celtic Connections artistic director Donald Shaw - to global success, bringing Celtic music to the world.
"We [Capercaillie] were just given all these opportunities, travelling the world, meeting people," recalls Matheson. "It was bang, bang, bang in the early days, not ever really taking time out to think, 'What's happening here? This is explosive.'"
Much has changed since eight-member-strong Capercaillie sprang out of Oban more than 30 years ago, not least the cultural renaissance of Gaelic and Scottish culture generally, which has seen folk music on the ascendant. Nor at 51 is Matheson the same person who was pushed into singing for an audience as a child, and has struggled with nerves and confidence issues throughout her career. No, the strikingly beautiful woman talking to me from the sofa of her PR's grand, open-plan flat in Glasgow's Park Circus, seems to not just be peaking, but scaling new heights. Last Sunday, she sang at the Glasgow Commonwealth Games 2014 closing ceremony, sharing a billing with Kylie, Lulu and Deacon Blue to deliver a haunting rendition of Robert Burns's Ae Fond Kiss in front of 40,000 spectators and a TV audience of almost eight million.
"What a night!" she says of the experience, admitting she was "nervous" about performing live to such a huge audience. "But thankfully," she adds, "I was singing a song I'd been singing for 20 odd years. The atmosphere at Hampden was of sheer celebration with the volunteers and workers, and I felt both proud and humbled to be part of it."
Things were very different in the band's early days, when, says Matheson: "It took me a long time to find my own feet. I feel like somebody else was putting me there - and thank God they did - but now I feel grounded in terms of my own ability. I'm aware of what I can do, what I can't do. The whole emotional journey of my life - I feel like I'm finding peace."
Part of that is getting older - "being comfortable with who you are and not trying to be someone else" - but it is also the experience of losing both her parents very close together a few years ago. Her father died suddenly in his sleep aged 77. Shortly after, her mother was diagnosed with cancer and given a year to live. Matheson and her siblings (two older brothers, one younger sister) nursed her for a year before she passed at 76.
"You are never prepared for that sort of thing, are you? It happens to people every day and there's a lot worse happening - but it's your own, and so it's trying to articulate that."
The experience was a "powerful thing creatively", and sparked Matheson into writing her own material - something she'd never thought she would do. "I've found myself sitting with a pen and paper and this stuff just comes out of me. It is as if it's uncontrollable, so I'm intrigued to piece it together. This kind of music, it's about what's in here," she says, pointing to her chest. "The driving force is internal and that's what's happening with the lyrics and emotions that are coming out at the moment."
Did she need that kind of emotional trigger? "Maybe," she replies. "The loss of my mother released a lot of stuff - maybe cultural hang-ups I had. To see, in her lifetime, the incredible cultural shift that's happened … it's an explosive time for Scottish identity. I realised I was holding back as well, in respect of her. Now I feel like there's this driving force in me where I want to get to grips with exactly where all this stuff has come from.
"I've started doing my family tree. My mother never spoke about her past and although it was a huge part of what I did, there were always areas that I couldn't really ..." She trails off.
Is there anger there? "An anger that my mother and that generation had to bear that cross [regarding Gaelic]," she concedes, "But my foremost feeling is excitement about where Scotland is at now."
Matheson, in many ways, has been on the frontline during our country's cultural shift (incidentally, she is voting Yes in the upcoming referendum), and her mother's death reawakened her own perspective. "It made me realise how fragile everything is - literally from the death of your parent to a whole culture. It [her death] was symbolic of the identity crisis we had as a people. It was a very powerful shift that I never really thought about or explored."
Prior to her mother's illness, Matheson had 75% recorded an album (she has had three previous solo albums), but she has only recently gone back to it.
"I'm looking at it with a totally different head now - I'm in a different place - and that's a good and a bad thing. But there's some brilliant stuff on it and I'm going to pursue it and get it done."
The album is a mix of Gaelic and English songs and even includes a Bob Dylan number; but Matheson is also embarking on a traditional album of "pure and ancient" Gaelic songs. The confidence to mix genres is indicative of Matheson's current self-assurance, but she also feels a responsibility to "get it right" for the new generation of Gaelic singers coming up, in part because of Gaelic's resurgence.
Of course, Matheson, and indeed Capercaillie, have been role models to a new generation of folk singers. Long before folk became "cool" again, the band produced the first Scottish-Gaelic records to reach the UK Top 40 and played around the world, from the Hanging Gardens of Babylon to the Copacabana beach in Rio de Janeiro.
IT all started in Argyll during the 1980s. Donald Shaw had started an instrumental band during the Mull Music Festival and when the BBC asked them to play, they needed a singer. Thankfully, Shaw knew a girl from the village ... The pair had been at primary school together; had played together in another band, The Etives, during secondary school. By the time Capercaillie got properly under way - and the couple got together - Shaw was 16ish, Matheson 20ish. Their first big break was a tour of Canada.
"And we all just ran with it, probably not thinking it would turn into anything major, just thinking, 'This is great fun, we're young.'"
Doors opened for them. They were in the right places at the right times. There was never a "master plan", but Shaw was always the driving force. Matheson was drawn to his musical drive and ambition. "I was the polar opposite," she muses. "Maybe that's what attracted us. His constant encouragement has been a big vehicle for me. For long enough, I wanted to be in the background, I was absolutely terrified - of interviews, photographs - and he said, 'Karen, you're going to have to do it at some stage.' The confidence thing was a real problem for a while, but he helped me through that. I used to get so angst-ridden and nervous. Singing on stage never came naturally to me - never."
Part of this was not being a native Gaelic singer, she says. "I always had a lack of confidence about what I was doing and whether I was doing it right."
It peaked in the early days, but it was cyclic. Just as soon as she thought she was comfortable, the anxiety would return. "But it's always going to be there," she states. "Gaelic is not something I grew up speaking so I'm always going to feel under pressure."
She admits to still getting anxious and nervous, but believes "it's probably part of the process of what makes me tick. If you become complacent, you would be scuppered, wouldn't you?"
As such, Matheson loves singing as much as ever. "Probably more," she smiles. "Because I've done the whole lack of confidence thing; I've done the whole 'not really knowing who I am'. I feel like I'm finally finding my own feet. Finally in my 50s, I'm gaining a wee bit of confidence on stage."
Ageing, she says, is something she embraces. "I see it as a gift that we're here. I've lost friends with children to breast cancer. It puts mum dying of cancer in perspective because she'd had a great life. She said, 'I've been so lucky' - that's how she left this planet. She taught us to die, for God's sake." At this point, a film of tears settles over Matheson's eyes.
Her mother, a housewife, was a "humble and sweet" woman. Her father, who was musical, worked at the local quarry. Initially they were terrified of their daughter taking off with the band and that feeling never really left her mother, despite her pride in Matheson's achievements.
For their own part, Matheson and Shaw put off parenthood; Matheson was 38 when she gave birth to their son, Hector.
"We dragged him on the road when he was a baby. But I feel privileged to have had the chance to take him with me. I probably saw more of him because he was stuck to my arm side-stage or sitting in his buggy while we were performing."
It worked for the couple because she and Shaw were both on the road. In fact, Matheson says could never have toured for years with the band, without her husband. Working together hasn't "all been a bed or roses", she concedes, "But neither of us is egotistical. We balance each other out. I never bear grudges and I can move on quite quickly in my head. You think, 'Right we've rowed about that, that was the music, now can we get back to what we're having for dinner tonight'."
It goes without saying that parenthood has changed Matheson. "There might be 5000 people out there wanting to hear you sing but your focus is still, 'Has this child been fed?' And I think that's a brilliant thing. You become less selfish. It's the best thing that ever happened to me, without a doubt."
The family live in Bearsden, near Glasgow, and sometimes, Matheson regrets not sending Hector to Gaelic school and laments Hector's urban upbringing, far from the roots to the past she and Shaw grew up with. But she hopes their musical and cultural influences have seeped into him.
"Recently," she says, "I've noticed he'll go around the house singing a Gaelic song he's heard or he'll have on Donald's Hebrides CD. But we're not meant to know."
One day Matheson would like to return to Argyll. After her parents' death, her feelings about home changed. "I think I'm still going through that period of transition of losing them. Whether it's about place, people or your identity, they are now gone and, for a while, I was just thinking, 'What's the point of going there?'
"But I feel a real link to the village. That part of Argyll kick-started everything I do and made me who I am. We'll end up back there … eventually."
Despite having now lived longer in Glasgow, Matheson has always felt as if she was passing through the city.
"I've never really settled here. As much as I love it, I'm a country girl at heart. Every so often I just want to run away."
Despite Capercaillie's success, there have been difficult times, including a nasty bus accident in their early days when their drummer broke his neck and they were all injured to some extent. There has been rivalry in the past. The members have all had their own solo projects, something they realised they had to do after years of relentless touring where they "played the game, wore the clothes, did the hair".
When that became exhausting and they realised their personality was being sucked away and felt that their record label, BMG, was trying to change them, the band moved to a smaller label and, after touring last year to mark their 30th anniversary album, they rediscovered the joy of playing together.
"We realised that's what it's all about. We could still be doing this in our 80s - going on [stage] with our Zimmer frames. We're lucky the kind of music we do is timeless." So there are plans to do another album. "We said 'no more', but the mood..." - of the country, that is - "is firing everybody and you have to run with that while it's there."
For her own part, Matheson feels privileged. "I've had this amazing journey, life and career, when essentially I'm just a wee girl from a wee village on the west coast that sang a few Gaelic songs."
Capercaillie play the Kelvingrove Bandstand on Thursday as part of Magners Summer Nights, with support from The Silencers. For tickets (£25) and more information on this and other concerts in the series, see www.regularmusic.com