Walk into the Kevin Kavanagh Gallery at the moment and, if you are not warned, your first impression might well be that it's occupied by an exhibition consisting simply of blank, monochromatic paintings. Just seven portrait-shaped works, to be precise, their flat, unvarying surfaces are either blue or grey. The atmosphere is subdued, calm, conducive to contemplation. It quickly becomes apparent that the paintings are not quite flat: their surfaces are evenly brushed, and there is a degree of tonal variation, more noticeable in some than others. Of course, if you've seen work by Paul Nugent before you'll already know what's going on.

The effect is clearest in the three dark blue pictures. As you look at them, you can see the ghostly, underlying image of a nun in each one. There is something incongruous and perhaps even slightly comical about this. In each case the pose and elaborate habit are almost identical, but it becomes apparent that there are several different nuns throughout the seven works in the show. Some of the underlying images are painted as if they are photographic negatives which, together with hardly any surface contrast at all, makes them difficult to decipher, but once you get the idea it's easy to make out each figure.

There are several ways of interpreting these various elements. Nugent might be offering a comment, ironic or otherwise, on the notion of spiritual content in abstract art, rather in the way that David Godbold has done. After all, what could be a better symbol of spirituality than a nun hidden in a painting? Or he may be alluding to the historical process of the secularisation of art, or simply making a pointed allegory about painting's capacity to embody several levels of content.

It's striking, though, that the work doesn't come across as being dismissively ironic. It is too rigorously pursued, too nuanced and obsessive in form, which makes its element of ambiguous humour all the more intriguing. Rather than thinking of figure and ground as opposites or alternatives, perhaps we should regard them as complementary. After all, the austerity of monochromatic abstraction could be seen as relating to the austerity of the nun's vocation.

Questioned on these and other matters, Nugent is quite forthright, though he emphasises that details of the work's making are just that: details of making. They are striking all the same. He's made 10 works in this series, though he originally envisaged 50. Four different people have acted as models. They are not nuns; some are fellow artists. He made the elaborate habit himself. "It's based on the Carmelite habit, and it's as accurate as I could make it." The process was, for him, a little like making preparatory studies. All the time he was homing in on what exactly he wanted. He used different sitters to "get away from a purely repetitive image, to create some breadth between them". There are on average, he reckoned, about two unsatisfactory figures underneath each finished figure.

Why nuns at all, though? "They relate to ideas of meditation. I was thinking about closed orders, where the nuns isolate themselves from the outside world, to create this meditative space. They're apart from the community but they would very much see it as working on behalf of the community, through prayer. I see those long hours spent in the studio in something like the same light. Making art, not for everyone, but certainly for me, is a meditative discipline too." And, while perhaps you wouldn't regard it as being done on behalf of the community in any obvious sense, in the long term artists do contribute to the culture.

So on one level the discipline and moral purpose of religious life serve as a metaphor for comparable qualities in the practice of art. He photographs his models, then makes representational paintings, in black and white, from the photographs. In the past he's done this quite straightforwardly. "This time, I dragged the surface slightly while the paint was still wet to unfocus the image just a little." Then, when the image is dry, the layers of over-painting are applied in thin glazes. "The paint is dragged from bottom to top, so you're competing with gravity. I have to keep working it until it's dry enough to stay."

He likes the idea of making us look again - and again. Not that everyone does look twice. One or two visitors have paused just inside the door of the gallery, glanced around, obviously thought "blue painting" and left without venturing any further. But once you start to engage with the work you keep going, drawn in by its subtleties and by the questions it encourages you to ask. The use of negative images places the figures at one further remove. Positive and negative combined would cancel each other out, would make, in a sense, the flat, uninflected surface that each work hints at being. "I also had in mind X-ray photographs of paintings," Nugent observes. X-rays reveal underpainting and might prove or disprove authenticity.

Mainly, though, he thought of the negative images as indicating transcendence. "Ultimately that's the aim of the sisters, that's what they hope to achieve through prayer and meditation." And, he suggests, that's what art tries to do as well. "There is this sense that painting must go further. There has to be an extra dimension." That's why he couldn't settle for a representational image per se. "I don't want to make images, I want to make paintings. I'd like to think that comes across - and that the gallery is a contemplative space."

Paul Nugent's new paintings can be seen at the Kevin Kavanagh Gallery, 66 Great Strand Street, until September 26th