`Willy nilly, I have become a painter of religious themes," jokes Patrick Pye. But this has been no random journey. He has aimed, with his paintings and stained glass, to achieve the definition of a sacrament: "an outward and visible sign of inward, invisible grace". He converted to Catholicism at the appropriate age of 33. He is now 68, mellow, with a bushy, grey beard and a throaty chuckle.

He lives in Piperstown, two miles uphill from Tallaght, Co Dublin, his snug house surrounded by frosted fields and lowing cows. He has lived here since 1967. He works in a studio which adjoins the house, via a low archway. I am warned by his wife, the weaver, Noirin Kennedy, that the strong aroma of wax in the studio may be overwhelming but in fact the scent is quite pleasant. The room is dominated by a huge window, upon which is hung two parts of the sanctuary window from his current stained-glass project: "This one is The Coronation of the Virgin. It's part of a big commission for a church in Cladymore, South Armagh. I have another artist, Paul Brooks, working on it with me." Wintry sunshine illuminates the marvellous alignment of blues and bright yellows.

Stacked in narrow pigeon-holes nearby are panes of mouth-blown glass, divided according to colour: turquoise, prussian blue, violet, ruby. A pot-bellied stove sends out waves of heat. It is fed from the heaped bank of coal in a nearby stone trough. Vases, pictures, postcards, letters and pieces of glass litter the room. A handwritten quotation swims into view: "By being the curator of our images, we care for our souls." The author? "Thomas Moore. He's a Canadian who trained as a priest but became a psychologist."

Pye was born in Winchester, Somerset, but was brought to Ireland at the age of four by his Irish mother when his parents' marriage broke up. They settled in Templeogue, Co Dublin. His mother's family had come to Ireland originally from Taunton: "They came to evangelise the Papists. I'm a sort of vengeance on them."

His mother was "an intellectual woman": "She taught piano and read books by writers like Huxley." They lived on very little, and Pye was able to board at St Columba's College only because his paternal aunts paid the fees.

With his mother, he attended an exhibition of paintings by John Piper: "There were a lot of paintings of bombed buildings in Britain, with wonderful pediments and classical facades. That weekend I painted a picture of Roman ruins and called it The Grandeur That Was Rome. I was 14. It was really a discovery for me. I had been imitating other artists up to this. This painting was the first that was in line with what was to follow. The image encompassed time, which was very important for me."

He found little in Anglicanism that appealed to his imagination and a major part of the appeal of Catholicism was his discovery of its medieval art. Another "revelation" was El Greco: "He swept me off my feet. It was a shattering experience. His style was so extraordinary - it was a revelation to me that painting could have such imaginative power." He has written a book about El Greco entitled The Time Gatherer (1991): "I'm still smitten by him, particularly his radical treatment of the Christian theme."

His own style bears something of El Greco's stark, dramatic power, particularly in a recent triptych entitled The Abandoned Boy, where he depicts God's demand that Abraham sacrifice his only son, Isaac.

However, in his teen years: "I didn't allow myself to imitate El Greco. I knew I'd come a terrible cropper if I tried to imitate his very sophisticated style." He modelled himself more on the "eloquent, direct narrative style" of late medieval artists such as Fra Angelico and Roger van der Weyden, and the Renaissance painter Piero della Francesca: "I imbibed quite a modern sensibility. I don't know how! I gravitated to artists who had been discovered in the generation before mine." Like those early artists, tempera is his preferred medium and his style has a similar, quasi-primitive quality.

He was less interested in religious art after the Renaissance: "the whole spiritual temperature dropped after that time. With the Reformation and the Enlightenment, people lost their hold on Christian truth and the imaginative world of the Middle Ages. The symbolic language grew rusty." He attended NCAD, where his lecturers included the sculptor Oisin Kelly: "I never followed a course all the way. I had to keep stopping to go off and earn some more money," he jokes. "I spent a year at the College of Art in Maastricht in 1958, and that was more formative. It was on the Continent and that was where modern art was happening. My teacher there had been a pupil of Heinrich Campendonk, a highly considered artist whose name I had heard in Dublin."

He moved to his present house from a flat in Fitzwilliam Square. He was delighted by the mountains, where he took long walks and painted a lot of landscapes. Injuries from a road accident yielded a sum of compensation (£500) which allowed him to purchase the equipment he needed to make stained glass in his studio: "I needed a kiln and a mill to stretch the lead." His interest in stained glass had begun while he was at the NCAD and he began making it in earnest from the latter half of the 1950s:

"What appealed to me most about working with stained glass was colour. If you can visualise the differences of tone that can be attained in a painting, the range (between one and 100) might be from 31 to 52. But the tonal range of God's own sunlight and the absolute obscurity of a bit of dark glass would be more like from one to 100. It is a very dramatic art form in that sense."

His main influence among the Irish, stained-glass artists was not Harry Clarke but Evie Hone: "I like the monumentality of her work. I like the Romanesque feel of it very much. She was very influenced by Irish Romanesque but she made it her own."

He has had many stained-glass commissions, including windows for Glenstal Abbey; St Mary's in Westport; Creagh Church, Ballinasloe; the Church of the Resurrection at Cave Hill Road in Belfast; and the Museum of Clongowes College. There was a brief fall in commissions during the 1980s: "Architects didn't want stained glass. It didn't suit their conception of the lighting."

He lived "simply" and supported himself by working part-time as an architect's librarian between 1960 and 1972. After that he spent two years teaching at the NCAD. He married in 1974: "I used to tell my students that if they wanted to be serious painters they shouldn't get married until they were 40. It takes at least 10 years to decide the sort of painting you want to do; then another 10 years to persuade everyone else that this is what you are most suited to doing." He laughs: "It was the only piece of advice I've ever given that I actually took myself." He met his wife Noirin - who was then proprietor of The Weaver's Shed - when he bought some tweed from her to give to a friend in Denmark.