Interview: Sarah McGuinness, singer and producer

• Northern Ireland-born Sarah McGuinness sports brown hair, but, as Sarah Townsend, Eddie Izzard's former girlfriend, has blonde locks

LAST week a dark-haired woman in a sequined dress was belting out showtunes at London nightclub La Pigalle. You may not have heard of the musician, Sarah McGuinness, but she has influential admirers.

Her new single, Mama Can You See Me Now, has been remixed by dance wizard William Orbit. And an incognito Eddie Izzard – in turban and glasses – applauded, before performing a DJ set in her honour. But who is Sarah McGuinness and why has a buzz been growing around the virtually unknown singer for the past few weeks? Earlier in the day I had discovered her remarkable story and pieced together her true identity.

Her background looks straightforward. She is listed as a composer mainly known for soundtracking Izzard's hugely successful videos over the past 14 years. She is also a singer and an actress. Pictures abound of her looking raven-haired and sultry. Yet there was something about McGuinness that didn't add up.

When I begin researching her ahead of our interview, I come across gossip on Eddie Izzard fansites. McGuinness may be Izzard's secret long-time girlfriend. Izzard has always remained tight-lipped about his private life, though the press has long tried to link him to women – to no avail.

Then I become confused. Further digging reveals that Eddie Izzard has two Sarahs in his life. I ask around. Some people think his girlfriend is actually an attractive blonde TV and film producer called Sarah Townsend. She is also credited with working on all Izzard's videos as producer/director as well as a writer and costume designer – clearly a multi-talented woman. Earlier this year she was Emmy-nominated for the film Believe: The Eddie Izzard Story, and there are pictures of her on the red carpet at the Los Angeles awards ceremony on her website.

Both Sarahs are multi-faceted and have known and worked closely with Izzard since the mid-1990s. But which is his girlfriend?

It is then that I make a bizarre discovery. Looking at pictures of the two Sarahs, I spot some unmistakable similarities. Could they be the same person? Does the woman in Izzard's life have, like him, a double identity?

It seems too bizarre to be true. I phone McGuinness's publicist and ask for confirmation of my hunch. The line goes silent. It looks like I am on to something. But which Sarah will I meet when I turn up to interview her at the Pigalle ahead of her show? I am greeted by Sarah McGuinness, 43, complete with messy raven bob. She has the remnants of a soft Irish accent. We talk about her music and the roles she played in the acclaimed Believe.

Filming the documentary was a labour of love, she says. They started shooting in 2003 with no budget, tracking down Izzard's famous friends George Clooney and Robin Williams, as well as interviewing family and schoolfriends. It's a fascinating portrait of a 15-year "overnight" success.

Oddly, McGuinness contributes interviews with herself in Believe – this time wearing an auburn wig. You can see why Izzard and McGuinness were drawn together: chameleon performers who constantly reinvent themselves.

I decide to drop the bombshell – which Sarah is she? McGuinness is momentarily thrown. Her manager is consulted and the interview is halted.

Then she agrees to talk more frankly, revealing the true extent of the extraordinarily complicated life she has managed to lead – and the major part she has played in the success of Eddie Izzard.

Not only are McGuinness and Townsend the same person, she is also Izzard's ex-girlfriend, she confirms. They met in 1989 when she was running a Fringe venue at the Edinburgh Festival.

Izzard – who admits he fancied her rotten –begged her for a slot. As she recalls: "He was very in-your-face. 'You're going to take me, I know you want to.' You're dealing with this fellow and you think: ' Who does he think he is?'"

But his charm, and stubbornness, prevailed. She took a risk on an unknown comedian. The rest is history.

SHE grew up in Londonderry in Ireland. Her family couldn't afford to buy records, so she listened to whatever was available in the local library: early Bowie, UK glam rock, Iggy Pop and Bond tunes.

She moved to London, aged 19, to study English and drama at Royal Holloway. She acted, directed Shakespeare and worked backstage in the West End, before setting up a comedy club, where she promoted comedians Ardal O'Hanlon and Jenny Eclair.

She took courses at film school and began shooting extras for Izzard's DVDs. But she always pursued music. Her first single was Mandy Says, produced by The Undertones' John O'Neill. When Mama Can You See Me Now became a cult hit in America, people begged her to release it.

Today, McGuinness and Izzard run a production company, Ella Communications, named after his beloved mother, Dorothy Ella Izzard, a midwife and nurse who died of cancer when he was five.

But music remains McGuinness's passion. She composed the soundtrack for Believe, including the upbeat Mama Can You See Me Now. "It's amazing how you can counterpoint the emotions in a scene and change the nature of what they mean," she tells me.

"But it is only in the final minutes you realise the song's significance. The revelation comes after a moment when Izzard finds a letter his mother wrote when he was a baby, where she refers to him as "Edward". Until that point, he hadn't even known she called him by his full name.

Izzard breaks down on camera when asked why he continues to push himself.

Why can't he relax, McGuinness asks. In an emotional confession, he explains it is a way to communicate with his mum. "I keep thinking if I do all these things, and keep going and going, then she'll come back", before he bursts into tears. It's a spine-tingling moment.

MCGUINNESS/Townsend was with Eddie Izzard when he came out as a transvestite aged 29. It was his courage about breaking the taboo that made him yet more admirable, she explains in Believe.

Today, they are close friends rather than a couple. She is over from LA for the Pigalle gig and to celebrate the launch of the DVD of Believe. But when it comes to her own music, she is determined to present herself as an artist in her own right. Hence the dark hair when she is performing as McGuinness.

She's not ashamed of her split persona. She claims it was hard enough to get funding in the arts as an independent female artist. Especially when you've had a famous ex. Yet she goes on to demonstrate how she can compartmentalise herself.

"I don't think the expressive part of myself is in any way connected with the analytical, obsessive production side of myself that gets the job done and makes movies. That's a totally separate part of me. I think possibly years ago that was the thing that tripped me up. Now I find the way to do it is to put them in two separate places. It's left brain/right brain split."

She's writing songs for a new musical by Tony Thompson, set around the stand-up circuit of the mid-1980s (a world she knows well). And making a solo album with tracks produced by Ed Buller (Suede) and Mickey Petralia (Beck). And, yes, she'll keep changing her hair colour to juggle the roles.

Directing is the day job, she says, but "making music is my passion, it gives me freedom and the lassitude to enjoy the things I started out loving. These days I go with being authentic rather than trying to be clever.

"I've just stopped trying to be anything I'm not," she smiles. "Just being honest resonates with people."

• Sarah McGuinness's single Mama Can You See Me Now is out this week; the DVD, Believe: The Eddie Izzard Story, is released on 4 December.