Veronica Dunne, "Ronnie" to the world at large, is a legend in her own lifetime. In her career as a singer she shared the stage with the likes of Kathleen Ferrier and Joan Sutherland. She is still active at 88 and has been a force to be reckoned with as a teacher for probably longer than she cares to remember: since 1962.

Singing and teaching are not a vocation for her but a passion that keeps her energised. When we meet, she complains about her limited mobility and requires a helping hand to stand up from a sofa. But when she’s in the whirl of term-time teaching, she says, the body moves so much more easily.

The Veronica Dunne International Singing Competition, which is held every three years, takes place from January 22nd to 24th. I ask her about the whole business of what a singing teacher actually teaches, and what advice she has for anyone appearing in her competition.

Talking to Dunne is sometimes like talking to a politician: every reply centres on what she wants to talk about, no matter the question. At other times, it’s like a masterclass in the minutiae of voice production.

A question about what you can and can't teach about singing leads to an answer about her teacher Hubert Rooney's views on the origin of bel canto. Bel canto, literally beautiful singing, is a term that's used with a multiplicity of meanings. Rooney, rightly or wrongly, traced it back to the Benedictines in Spain, and the explanation involved the different ways in which words such as casa and campanella are pronounced in different parts of Italy: in the north the "c" comes out like "h", but not in the south.

Musicianship first

First encounters with a new student are obviously crucial. Dunne puts musicianship first, although when she was very young, she says, she was no shining light.

“My mother used to send me up to do my piano study, and I’d jump out the window and go and play. To this day, I remember her saying to me, ‘One day you will regret this, child.’ And I have regretted it.”

Then, of course, there’s the voice. How good is it? What’s its range? It’s amazing, she says, how many female voices in Ireland are mezzos rather than sopranos.

After musicianship and voice, things become more intense. “You say, ‘You want to come and study?’ And they say, ‘Yes.’ You say, ‘You realise what it entails? You have to be a first-class musician. If I say jump, you say: how high? You have to dedicate yourself. You have to learn how to be a good actress. After that you have to learn definitely two languages, German and French. Italian is easy if you have the other two. But if you have a third one, it’s a plus.’ Then I say, ‘You must have the perfect figure.’ ”

The debatable advice on appearances might have a realistic if worrying foundation based on the preferences of some modern-day opera directors. But the advice about learning German is certainly based on pragmatism. “I do emphasise Germany. They have 60 major opera houses.” There’s the plus of speaking the language of the country, and she refers also to the practice of singing foreign operas in German, much as Opera Theatre Company and English National Opera perform foreign operas in English. Italian composers might dominate the popular end of the international repertoire, but Germany is the biggest market in Europe for the performance of opera.

Alarmingly direct

Dunne illustrates almost everything with references to singers she has taught, and she can be almost alarmingly direct in her appraisals. Fondness for the individuals and pride in their achievements don’t stand in the way of nuts-and-bolts analysis.

The first thing she teaches is how to breathe. “You don’t suck in all the air you can suck in. You just lean on it with your ribcage. And by degrees, as age comes on, your ribcage gets wider and wider and wider. If you learn how to breathe properly, you learn how to sing.

“It’s like putting a cigarette on an ashtray. You see that flow of smoke that’s never been disturbed. You as a singer have to create that in your body. You push your ribcage out, and then you bring your diaphragm up, slowly, slowly, slowly. You mustn’t go outside your mouth with the air. You’ve got to stay in the cavity of the head.”

The tongue, she says, “is the most important instrument you have in your mouth. It directs the sound to where it should go.” The sound “ee”, for instance, is created with the tongue towards the top of the mouth. She demonstrates, singing “ah”, “eh”, “ee” in sequence.

“ ‘Oh’ is your top lip,” she says. “ ‘Uu’ is your bottom lip, like the monkeys. The bone structure is so important, the loose jaw and the loose head. This is the only part of you that works [she pats her diaphragm] and the air comes up. When you’ve got the mechanism working down here it’s so easy.”

She brings to mind a class I had when I played euphonium in the school band. A teacher was brought down from Belfast, and he had someone lie flat on a bench for us all to watch the way the body moves when you breathe properly and how it moves when you don’t. It was a revelatory moment.

“You feel your ribs,” says Dunne. “You just push out. The more you can push out, the more you can control the weight of air. By controlling the weight of air, you can place the voice wherever you like. Your chin is very important. It must go back into the ear, and the roof of the mouth comes forward, so that you can open up the back passage for your top notes.

“Your top notes – if you’re on an airplane and you unlock your ears, that’s where your top notes go. You have to have the weight of air and energy in the body to hold that air, which is trying to throw itself out. The biggest sound you can make is through the smallest spacing in your bone structure. It’s frightfully interesting. I could spend a year or two on technique to get those things right.”

Dunne’s advice to her students extends beyond singing and into eating, drinking (“never get drunk”), dressing, etiquette, relationships (she recounts helping someone write a letter after a break-up), behaviour with colleagues (never make a negative comment, never divulge your own private information), and money (singing is an uneven career; you need to be ready for the low spots and never cede control of money to your spouse).

Working with conductors

She returns to the advice about musicianship and puts it in the context of working with conductors. “The first time you make a mistake with the orchestra, fine. The second time, the conductor is quite angry. And the third time he’ll tell anybody else, ‘Don’t engage her.’ You’re out of a job. So, which do you want to be?”

And the competition itself? She recalls the 2013 winner, American soprano Nadine Sierra. "She came in and sang the first round. I said to myself, 'God, she's great.' I came across her in a corridor before the second round. She was looking for somewhere to warm up. I found a place for her, and said, 'Do you know what? You're the winner.' She said, 'You can't say that.' I said, 'You're the winner.' "

So, how did she know? (Dunne is not on the jury.) “It’s instinct. She was the full package. She had her music studied to a T. There wasn’t anything you could query. And she had sincerity.”

And her advice to this year’s competitors? “All you can do is wish them good luck and say, ‘It’s your moment on that stage. You’ve worked for it. And go out there and sing from the heart. If you sing from the heart and you want something, you’ll get it.’ ”

• The Veronica Dunne International Singing Competition is at the Exam Hall, Trinity College Dublin (preliminary rounds January 22nd-24th) and the National Concert Hall (semi-finals 25th and 26th, finals on 28th). vdiscompetition.com