Theext Provost of Trinity, John Hegarty, is different. It is not just that he is a Catholic (the present Provost, Tom Mitchell, was the first Catholic Provost), not even that he is a former candidate for the priesthood. It is that the persona is so different to the Trinity College one.

He still has a trace of a Mayo accent, his hand gestures and general body language and the inflections in his speech are very much those of the repressed Catholic of the 1950s and 1960s (like many of us). But, unlike many of us repressed Catholics, there is an appealing openness about him.

There is no pretence, no self-importance, no grandeur, no conceit. There is a passion, almost an innocent passion, for knowledge, for discovery, for life.

The interview took place last Wednesday afternoon in his room in the physics department at Trinity, at the back of the rugby pitch.

Indeed, it was hardly an interview, until close to the end, for he spoke volubly and for long stretches, unprompted, about himself, about his "vocation" for the priesthood, about his loss of religious faith, about his research, about the science of lasers (his speciality), about the future of science and philosophy.

Only when he was probed about Trinity's role in the community was he discomforted. And he was then discomforted a lot. After the interview ended, he returned to the issue. Defensively.

He was born on a 40-acre farm outside Claremorris, near Knock, in 1948. He was the youngest of five children. His father was 69 when he was born and died when he was 12 years old.

His mother, now aged 91, is still alive and is proud of his election as Provost of Trinity. (He won the election last Saturday by a comfortable margin, winning 40 per cent of the votes on the first count and 68 per cent on the final count.)

He went to the diocesan secondary school, St Colman's in Claremorris, and while there decided he wanted to be a priest.

"The priesthood was seen to be the highest form of life. I suppose I saw it as something that I really had to do."

He is quite unselfconscious about this period in his life. "When I went to Maynooth I took it very seriously, probably too seriously in fact. The kind of life was like trying to be as perfect a human being as possible.

I found it wasn't possible." He says he had doubts about his "vocation" from the day he went to Maynooth. He found the regime stultifying. "It attempted to get rid of your individuality and I found that very difficult." It was almost a monastic life.

Asked if he was lonely then, he replied: "Very lonely." It was in his fourth year (and, as it turned out, his last) as a seminarian. When he was doing theology, his doubts crystallised. "It was then that I began to question religion, not just my vocation. I began to question religion - big time. I found that the only way I could see myself moving forward actually was to ditch the whole thing."

He didn't talk to anybody about it because the thoughts in his head were "off the wall". Only in his last few months there did he talk to other seminarians and found he was not alone, that several others were also having doubts not just about their "vocation" but about religion.

In that last year as a seminarian, he became interested in cosmology (he did a primary degree in philosophy and science). "Cosmology (the science of the universe) is the other side of the coin (to religion). You leave God out of it and you get interested in how it all began, the big bang, the idea of continuous creation, where it is all going to end. I found it equally satisfying, deeply satisfying [in contrast] to the religious explanation."

It was in April of that final year he told the dean, Mgr Michael Olden, who later became president. He had a sense the college authorities were relieved to hear of his decision to leave.

He stayed until June in the seminary and then went to England and worked as a manual labourer, laying drain pipes. He had been offered a teaching job in his old school, St Colman's in Claremorris, so he went back to the "lay" side of Maynooth to do a Higher Diploma in Education (the college had just then begun to admit lay students, including women. As a seminarian he had a crush on one or two women in the college but won't now say who they were).

He became absorbed by physics, so he "ended up" doing a Ph.D in UCG. The main reason he wanted to do science was because physics was asking, again, all the great questions about existence.

His Ph.D subject, in a specialised area, concerned light, which wasn't "the ultimate question" but fascinating anyway. The choice of subject was influenced by a young physicist, Frank Imbusch, who had just returned from the US and had worked with the man who had discovered the laser, Arthur Schawlow.

"There were all kinds of new ideas about light, what you could do with light, how patterns of light hang together, what made atoms hang together. You shine light into a material and either it goes through or comes back out and when it comes back out it is modified by what is happening inside the material.

It's like bringing out information about the material." He took four years to do the Ph.D, finishing in 1976. He enjoyed UCG hugely. "I was wide open for it, I was older as well. I was ready for it, more than ready for it. I absorbed it completely. I did lots of other things besides, I got heavily into photography. I was the college photographer for the student magazine. I'd go to every gig in Galway." He took part in a few student demonstrations demanding higher grants.

While in Galway he met Neasa Ni Chinneide, his future wife, now working with RTE. She is from west of Dingle. She worked in Radio na Gaeltachta as a student and now works in RTE television, as head of Irish language and features (she is on leave of absence at the moment).

She had done a masters degree in Galway in geography and was offered a grant to do a Ph.D in Madison, Wisconsin. He had a contact there and through that was offered a post-doctorate position in the University of Wisconsin in Madison. They went there in January 1977, having married the previous year. There he discovered "real research" and at a different level.

After three years there he was offered a job by Bell Laboratories, the most prized physics research centre in the world. "It was an incredible experience. There's always people better than you. It was a great place to be." He spent six years in New Jersey doing research on optical fibres, "the business of how you send, transmit information at high speed, via cables".

"It was an experiment I did in 1984 which sent more information on an optical fibre than had ever been done, using a technique that has now been commercialised." During this time they had a child and were "pretty much settled down" even though "New Jersey is a terrible place to live".

They now have two children, boys, one aged 18 and the other 10. Dan Bradley, then Trinity professor of opto-electronics, had a stroke in 1984 and retired. He was invited to apply for the position. At first he was not interested but "it set me thinking about the long term . . ."

"The idea of coming back to Ireland started to appeal to me and having children living in a culture that was not foreign to us (in Ireland). I came back and visited here a year later and there was a buzz about the (physics) department. I decided I'd go for it and I did. I came back another year later in 1986."