When you think of the biggest pop stars in history, a classically trained singer from County Donegal whose ethereal albums feature a blend of English, Gaelic, Japanese, Elvish and Loxian, the latter a language invented specially for her, is not the first person to come to mind.

You certainly don’t think of a woman who doesn’t perform live, takes three years to make a record and eschews celebrity for a life of seclusion and study at her castle outside Dublin (named Manderley after the house in Rebecca).

However, Enya — with album sales of more than 75 million, four Grammys and a 1988 hit, Orinoco Flow, that is synonymous with commercial Gaelic mysticism and New Age stress relief — is the most successful Irish solo artist bar none. She is also the richest female musician in the British Isles, with an estimated wealth of £90 million.

We’re at the Grand Hyatt Hotel in Berlin, a vast place with a suitably moneyed but Zenlike feel. In a few hours Enya — born Eithne Ní Bhraonáin, anglicised as Enya Brennan — will launch her eighth album, Dark Sky Island, in which she sticks to her formula of gossamer harmonies, strings, synthesizers and escapist production.

She is in one of the larger suites with Nicky and Roma Ryan, the husband-and-wife duo she has worked with throughout her career. Nicky, a tall man in a bright blue suit and with a large, silvery moustache, is a former sound engineer who once managed Clannad, the New Age/Celtic folk group that features three of Enya’s eight siblings and two of their uncles. He is responsible for the layering that gives Enya’s vocals their otherworldly quality.

Roma is a petite, raven-haired woman who writes all of Enya’s lyrics, even those in Loxian. After brief introductions they disappear leaving Enya, who is a birdlike, youthful 54 in a fitted red and black dress, to glide into an armchair.

I ask her about her typical working day. “It’s very much nine-to-five,” she says. Polite and earnest, she speaks in a mellifluous, middle-class Irish accent. “I leave my castle to go to the studio, which is on Nicky and Roma’s grounds, and begin by working on the melodies. I’m a slow composer.”

You can say that again: Dark Sky Island was seven years in the making. She explains how, once a melody is ready, Roma will respond to it by working on the lyrics. Nicky and Enya will then chip away at the production for months on end, layering her voice until it achieves that celestial harmony that has proved so relaxing in health spas the world over.

“We might record 200 voices for one song and erase them all if we don’t feel they’re working,” she says. “If I haven’t come up with anything we can use in a week or a month I do begin to feel anxious, but every day I get closer to the melody I’m looking for.

“It’s an emotional feeling. I don’t think, ‘I’m going to write a piece about this subject,’ I let it evolve naturally. I don’t know if it will be an instrumental, a song in English, Gaelic, Latin, Loxian . . .” Loxian, it turns out, is more than just a language, it’s a mythology, based on the idea of a futuristic Celtic race that leaves Earth for a planet near Aldebaran, a giant star 65 light years away in the constellation of Taurus.

“It goes back to The Lord of the Rings,” says Enya, reasonably. “It’s a book I read as a teenager and loved, and Roma wrote May It Be [Enya’s Oscar-nominated song for the 2001 movie The Lord of the Rings: The Fellowship of the Ring] in Elvish. Then we were working on the next album and we could not get any language to sit with it, so Roma suggested writing a language in order to make the warm vowel sounds I wanted.

“She started putting stories behind it so I knew what I was singing about, and Roma felt the Celts became the Loxian people. On the new album The Forge of Angels is the name of their spaceship and Loxian Gates are what surrounds the planet. This is in the future, of course, so we don’t actually know what planet they end up on.”

You might expect this kind of thing from a teenager who views the 20-sided dice in Dungeons & Dragons as the pinnacle of human achievement, but not a millionaire. I ask Enya if she sees a spiritual element in her music. “People say I’m a very spiritual person,” she confirms, with the slightest of nods.

Perhaps that spirituality began with the single biggest influence on Enya’s life: boarding school. As the sixth child of nine (she has four brothers and four sisters), she says she had to fall in with everyone else until she was 13, when she went to a convent school in the County Donegal town of Milford that was run by Loreto nuns. She experienced freedom for the first time, whatever the image of Irish convents might suggest to the contrary.

“Coming from a family where you were not asked a question because the decisions were made for you, it was wonderful. On the first day they asked us who would like to study piano, who would like to be in the choir and who would like to study music. I put my hands up three times. It was a dream come true because I heard my own voice for the first time. After that I realised I could make my own decisions.”

Enya was 18 when Nicky Ryan suggested she join Clannad, and when Ryan left the group two years later he took Enya with him. After writing music for the 1986 BBC documentary The Celts at the suggestion of Roma Ryan, Enya had a huge breakthrough in 1988 with her debut album Watermark and its single Orinoco Flow. There began the Enya-styled route to New Age success: no live shows, little publicity, just her and the Ryans in their castles outside Dublin, making music together in secrecy for years on end.

“I felt I had a choice to do what I was comfortable with, which goes back to my independence at boarding school,” she says. “So I walked away from the things that would have made me more famous. You don’t have to seek fame to be successful. After Orinoco Flow I realised the albums would suffer if I became bigger than the music.”

Not falling out of nightclubs or selling pictures of your living room to Hello! magazine is one thing, but never playing live is a rather extreme way for a singer to avoid the pitfalls of fame. Enya claims this is circumstantial as much as anything, explaining how her record company was not initially prepared to front the cost of sending her out on the road with an orchestra and a choir, the only way she could replicate the sound of the albums faithfully. Then the vast sales of Watermark meant the slow, laborious process of recording albums took precedence over concerts.

“I was on stage when I was three and a half years of age, in a Christmas pantomime in Gaelic in the local theatre in Donegal,” she announces, as if to suggest that I must be mad to think that her not playing a single concert in her 31-year solo career might be down to stage fright. “It was a beautiful, magical scene. There was the Easter Bunny, Santa Claus, the Snow Queen . . . and Little Red Riding Hood.” Was she, I ask, Little Red Riding Hood?

Fixing me with a stare of truly Celtic intensity, she replies in little more than a whisper: “Yes I was. And there were Gaelic singing competitions in Donegal and Derry, and each one of us in the family would enter them in our own age category and come home with medals. My dad has toured and sung all his life. My mum is a music teacher. It doesn’t feel strange to be on stage.”

After an hour with Enya, it doesn’t feel like her enigma is lessened in any way whatsoever. Her demeanour, regal but strangely childlike and unworldly, fits in with the ethereal quality of her music. She is single and, as far as I’m aware, has never been linked publically with anyone. I ask if she made a conscious decision not to marry or have children in order to concentrate on the music.

“I never sat and thought about it,” she replies. “If it happens, fine. If it doesn’t, that’s my decision. I never thought, ‘Oh my God, I’m a woman, I should have children.’ ” She reveals that she did have a break after releasing And Winter Came . . ., her album of 2008, spending a year buying a house in the south of France and working on its decor, but otherwise she offers little about her personal life. Everything we need to know, she explains, is there in the music.

“I am a very private person,” she says, “but I feel I’m revealing through the music. You can hear my spiritual side, what I regard as important. In that sense I’m open to the public.”

It can be hard, though, to know what somebody’s songs are about when she sings them in Loxian. “People adapt the songs to their own emotional story regardless of what language I’m singing in,” she says. “I was speaking to fans in Tokyo and they said they hear the emotional content in the melody. People adapt the song to their own story.”

Then she concludes — fittingly for someone who sings in a language spoken only in a castle in Ireland and on a planet in the constellation of Taurus — “In that way, the songs are truly universal.”

Dark Sky Island is released by Warner on November 20

The London Times | 2.11.2015 | Will Hodgkinson