Early in December, the Model Arts Centre in Sligo will close for extensive refurbishment. Originally, this was supposed to have happened much earlier in the year, but delays allowed the centre to continue functioning - and left it with the problem of last-minute programming. To its credit, it didn't just fill the blank spaces, but continued as it had begun, with a series of thoughtful, substantial, carefully installed exhibitions, ambitious projects that confirm the sheer potential out there for enterprising curators. The centre concludes this prolonged spell of injury time with an outstanding show, Paul Mosse's Personal Space. As with other, comparable shows mounted there, it is something of a retrospective survey. The earliest piece dates from 1981, the most recent is an unfinished work-in-progress. The show confirms Mosse as an intriguing artist, one whose work reveals additional layers of subtlety each time you see it.

It also suggests an interesting progression. He was born in Bennettsbridge, Co. Kilkenny in 1946, and studied first in Berkshire and then at the Chelsea School of Art in London. He now lives and works in the country, close to Inistioge. Drawings from the 1970s, not included in this show, are obsessive, minutely detailed accounts of aspects of - more or less - the natural world. Views of gardens, for example, or a fragment of a bat are plotted in such meticulous, exacting detail they become something else, something virtually abstract. It is as if, rather than trusting to sight alone, Mosse is subjecting everything to a minute examination, through several levels of scale, or as if he is combining microscopic with infra-red with taxonomic with chemical analysis, and coming up with an entirely novel way of looking at things. It's close to impossible to convey the nature of his more recent work in reproduction. A typical large scale painting is a mass of colour, often garish, dolly-mixture colour, that looks as if it was created by an artistic relation of Jackson Pollock. The surface is likely to be rough, comprising not just blobs and masses of pigment, but also what look like collaged scraps of plywood, and the ragged depressions of holes cut into plywood, and heaps of sawdust, and probably other bits and pieces, like nails, or broken drill bits or match sticks. So that while he is certainly a painter, the term "painting" is not quite an adequate description for what he makes.

It's not just that much of his work is sculptural, built up in dramatic relief or hacked through. It's more that his exhaustive, precise methodology suggests that what he does falls somewhere between conceptual art, process-based painting and sculpture. Yet he is wary of such labels himself, tending to deflect attention away from procedures back onto the finished art object. That, he believes, must stand or fall on its own merits, without special pleading. Fair enough, but it's worth considering his working methods for a moment. These partly entail enshrining arbitrary rules, actions and motifs as absolutes - an area rich in metaphorical possibility. Throughout long, elaborate stages, he repeatedly deals with successive given sets of circumstances, each determined by a previous move. Each stage can be extremely prolonged, often involving repetitious, laborious tasks, giving the process a ritualised, meditative quality.

When we see the work, because of the sheer level of detail involved, we know that it represents a considerable investment of time and effort, which is something worth bearing in mind. But Mosse is quite right to insist that it be judged on its own merits. Time spent is in itself no guarantee of worth. The paintings and drawings look abstract, but there is usually a representational motif concealed somewhere in there, however invisibly. It doesn't matter what it is. In fact, some years ago he more or less gave up using new or different motifs, and has used versions of one template as starting points. This coincides with what might be described as an increasing openness in the work. Untitled, from 1987 is typical of many works of the mid-1980s in that it is a beautifully balanced, harmonious all-over composition. It is pretty much perfect, and you wonder how the artist could surpass it. Tiny linear networks of about six colours, applied by syringe, are woven into a breathtakingly detailed network, like a coloured wiring diagram of the universe. On one level it is chaotic, a random mass of squiggly lines, but it resolves itself into a transcendent order. It is a complete world in itself, a world at equilibrium.

Other works duplicate this effect, and more come close, but they seem to represent a relatively brief phase in terms of Mosse's output, particularly considering how painstakingly slow he is to finish pieces. A comparison of anything from this period with a painting made over the last few years is salutary. Now, each painting is still a world of its own, a scale model of its own reality. But that sense of poised perfection is gone. The smooth, all-over symmetry has broken down, and not just broken down once, but collapsed like a deck of cards, opening a whole series of disjunctures and gaps. Now there are arbitrary divisions that divide the surface, and, in accordance with the logic of that initial, arbitrary fracture, objects, colours and materials accumulate in some areas and not in others. There is an altogether different feeling to the images, an openness to the imperfection of things, to contingency and chaos. These works answer the question of how Mosse might follow his vision of equilibrium. He followed it by shying away from the stasis of perfection, by allowing a serpent into Eden. It is as if all his recent work results from the introduction of a tiny disjuncture and rigorously pursuing its implications all the way down the line. In their richness of detail, the paintings describe a separate, analogous reality, a reality as arbitrary, as complex, as strange and as compelling as the world outside.

Paul Mosse, Personal Space continues at the Model Arts Centre, Sligo until November 31st, at least.