Five-year-old prodigies on the piano or violin are ten a penny by comparison with 15-year-olds who can conduct with any degree of success. What you have to do to become an instrumentalist - or, more accurately, what your parents have to do - is fairly clear. What you have to do to become a conductor, or even have a hope of becoming a conductor, is something of a mystery.

You can, of course, become a conductor by first becoming a pianist or a violinist, or even a singer, then trying to switch roles. You can train as a conductor at music colleges and conservatoires. But a good conducting technique is not a prerequisite for a conductor in the way technique is for other performers. The man - and it usually is a man - on the podium doesn't do the playing or singing, after all. It's enough to be able to make other people do it, whether or not you do it by the book.

Gerhard Markson knew in his early teens that conducting was what he wanted to do. "There was a development, around the age of 14 or 15, when it became clear to me that I had to become a conductor. I grew up with the piano, and I realised at a certain stage that what the piano could give me as a sound was not sufficient for me." Thereafter, conducting was an inevitability.

Along with the piano, the young Markson studied the organ. This led to an opportunity to conduct the church choir. It seems hardly surprising that unaccompanied choral music satisfied him little more than did piano music. So he engineered opportunities to conduct with choir and orchestra, beginning with the Hallelujah Chorus from Handel's Messiah and graduating quickly to a Mozart Mass. But that wasn't the definitive move. Markson didn't shift suddenly from being an aspiring professional pianist who conducted a choir to being a conductor with a busy career.

He muses on the surprising openness of the conducting profession, which allows in what are essentially unqualified practitioners, explaining it by the simple fact that orchestras now play so well. "We are living at a time where the orchestras are so good in technical terms that no matter what, no matter who is standing up front, they will deliver."

He describes himself as highly critical of his profession. "I have a very strict idea of what conducting is, and that's the Markevitch idea. There was one main statement: 'if an orchestra without you plays as well as they play with you, then you are superfluous as a conductor.' Which means my job is to make an orchestra play better than they would without me."

The Russian conductor Igor Markevitch, in his youth a protΘgΘ of Sergey Diaghilev (but in the role of composer), has been the most important musical influence on Markson's life. The route to Markevitch was rather circuitous. "The older I get, the more I don't believe the luck I had. My parents insisted that I study something 'serious' before I became a conductor. So I studied to become a music teacher. This wouldn't have been lost time anyway: it's a general study of music. But I had to study another instrument, apart from the piano, and, at the age of 22, I started the cello. My teacher was a very interesting young man, an early-music freak, with whom I spent more time talking than playing. So we had a very interesting time together."

It was the quality of their conversations that prompted the teacher to think of Markson when he was asked to find a German home in which the son of a family connection - Markevitch - could stay and continue to have piano lessons while he learned German.

"That changed my life overnight. Because, otherwise, my normal life would have continued. I would have done the conductors' class. But I would have gone just the German way. I would not have had this profound technical training which I got through Markevitch, the influence of this polyglot person that he was, intellectually the representative of a certain time, and a certain thinking in terms of music, of what music is all about. It just changed my life totally. It was very clear that had I not decided to become a bad cellist, I would never have had this fantastic opportunity."

Markevitch invited Markson to his conducting courses in Monte Carlo. "It was also through him that I went to Santa Cecilia in Rome, under the legendary Franco Ferrara, which gave me a year of three times a week conducting the Orchestra of Santa Cecilia. That was such a head start in terms of experience. When I came back to Germany to work in a theatre, it took just four months before they let me in the pit - normally it takes two years - because they knew I had been with Ferrara in Rome. I conducted Wildschⁿtz by Lortzing by heart. I'd done weeks of coaching work on it, and that was how it really started."

That was January, 1975, in the Bavarian city of Augsburg, where, in standard German style, he had to take over in the middle of a run of performances without any rehearsal. It's a sink-or-swim challenge, and he passed with ease. And, having journeyed through the lower echelons of the German opera and operetta repertoire, just two years later he conducted his first professional symphony concert.

As a young conductor, the main challenge was "basically to get through". It's a daunting business to take over an operatic performance that already bears the musical stamp of someone else. Can you risk trying out your own ideas when you've not had a rehearsal? Equally, can you risk not trying out your own ideas and missing out on discovering how viable your approach would be?

"The evening is your evening, and you check out how far you can go. I remember vividly the conductor who did Wildschⁿtz: nice guy, made good music. But I heard it differently inside. How far can I go without risking that the whole thing will collapse?" Can the tempo be altered here and there? The rubato adjusted? Will the singers go along with it? "That's exactly the point at which the conductor starts. You then make your performance, and not just a copy. Otherwise, why the hell do you conduct, if you don't have an inner picture, an inner sound, an inner tempo, whatever?"

As a young man, Markson was, he says, a Leonard Bernstein freak. Now, at the age of 53, he says he can see how, coming from a strict, Catholic, middle-class family, he was attracted by the glamorous opposite he saw in Bernstein. The Bernstein shadow-boxing style was hardly the approach he was going to pick up from Markevitch.

"Could I have been any luckier? You see, here's this young, strictly educated, rigid, inhibited man, and his ideal is the flamboyant Bernstein. And the next person I meet is Markevitch: absolutely clean in style, high mental discipline, move only what's necessary, don't make showbiz. A great school."

In the course of his early opera work, which included performances of Alban Berg's Lulu, Richard Strauss's Der Rosenkavalier and Luigi Dallapiccola's Ulisse, all without rehearsal, he made "millions of mistakes", through which he forged the focused rehearsal technique that wins him compliments today.

"I can't even start to try to define the moments of deepest desperation. How many moments of fear at night? You can't even imagine ever coming to the end of some of these operas without falling into a deep hole and making an idiot of yourself. So it's a hard school.

"Three times in my life, maybe every 10 years, there have been moments where I thought: 'that's it. I can't do it.' It was not so much a question of fear. It was a question of - it's hard to say in a foreign language - self-definition. Why do I conduct? Who needs me to conduct? What do I really want from conducting? I think it's basically a question that everyone who deals with art, in the one or the other direction, will always come to."

In the day-to-day business of conducting, what he now finds hardest is "to be the measure. That's what you have to do with all your question marks in front of the orchestra, and in front of the audience. You have to have a very solid opinion and security that that's how it has to be, knowing there are a thousand other ways to do it. The hardest thing in conducting is, within all these question marks, to have a spot and say: 'that's where I am.' "

With the National Symphony Orchestra, Markson sees the big challenge as being "to find the speech, mainly to find the sound". He later refers to developing a "handwriting" in sound. "It's a very short answer and it's very complex, because the only reason to become principal conductor is to produce a sound that belongs to the orchestra and you. Otherwise there's no point. To create your personal style and sound, it takes some time.

"The idea of becoming principal conductor of an orchestra has always to be that, whenever you leave, the majority of people that have dealt with you, in terms of playing, audience, you name it, should have the precise picture that it's better than when you started. If you leave and people say nothing has changed, or it's worse than it was before, that's bad news."

He talks passionately about working on Mozart and Haydn and improving the performances of contemporary music as a direct result of the way the 18th-century repertoire is approached. His consistent advocacy of contemporary music is, however, clouded by his idea of presenting a weekend of the Second Viennese School next February, as a sort of flag bearer for contemporary music. It is, after all, a full 50 years since Schoenberg's death.

But he sounds altogether more persuasive in his enthusiasm for Ian Wilson's Violin Concerto, which he premiered at the NCH in January. And he was clearly pleasantly surprised by the positive reception accorded to Pierre Boulez's Notations in April. Sadly, however, there's no sign of any effort to build on the achievement of the Boulez in the forthcoming season, where the emphasis is on Mozart and Strauss, and the non-Irish new music is not in the Boulez league.

The overall impression is of a man who's still trying to discover what the relationship is between the music he loves, the orchestra he's going to work with and contemporary Irish society. Lots of the questions I asked drew blanks, simply because he hasn't yet been able to get all the information he feels he'd need in order to have a cogent answer. This fits in with his philosophical position: you have to have the courage to face into a never-ending openness, and deal with the fact that the more you know, the more you realise what you don't know.

"It takes a lot of personal strength and integrity to deal with this and to get something positive out of it. This question mark is part of the energy source you get to find the answer - hopefully - knowing you will never find it."

Gerhard Markson conducts the first three concerts of the National Symphony Orchestra's new season at the National Concert Hall, in Dublin. The first is on Friday, with the second and third on September 14th and 21st. The opening programme couples Mozart's Jupiter Symphony with the Alpine Symphony by Richard Strauss