Eileen MacDonagh's new sculptures demonstrate the beauty of polygons and polyhedra, writes Aidan Dunne.

One geometric form predominates in Eileen MacDonagh's exceptional exhibition of sculpture at the Model Arts and Niland Gallery in Sligo. It is the icosahedron, one of Euclid's five regular polyhedra, and MacDonagh is eloquent in its praise. She's not sure when her interest in this complex geometrical figure tipped over into something more serious, but tip over it did. To the extent that she resolved to make a series of sculptures in which the icosahedral form is a constant while the stone that constitutes it is varied.

Standing beside one of the beautiful, broadly spherical forms, she explains that she feels there is something magical and miraculous about it, that it is composed of 20 equilateral triangles, and that it incorporates the other four regular polyhedra: the cube, the tetrahedron, the octahedron and the dodecahedron.

Regular polyhedra are composed of regular polygons, figures in which the length of the constituent sides is uniform, as are the angles they form. Euclid brilliantly reasoned that these five solids represented the entire geometrical repertoire of regular polyhedra.

Impressed with the elegance both of his thought and of the forms, Plato elevated them to the status of ideal solids, assigning each of the four elements a polyhedral form, with the octahedron providing a shape for the entire universe. Particles of water, he said, are icosahedral. As it happens, icosahedrons are not found in natural crystalline structures. Nor are dodecahedrons, though close, non-regular versions of both, and combinations of the two, are naturally occurring. MacDonagh is very taken with the idea that this complex form, a product of pure geometry, turns up early on in different cultures. "There are even," she says, "versions of it in objects found in Neolithic tombs in Scotland."

She is a tall, athletic-looking woman who positively radiates enthusiasm not only for what she is doing but for the underlying ideas behind it, and for everything that informs it. She was born and grew up near Geevagh in Co Sligo, going on to study at the local RTC. The art department there was small at the time, she recalls, and the personal input of just a few people, notably John O'Leary, Nuala Moloney and Fred Conlon, was crucial. "Fred Conlon, who died recently, was my teacher there. He became a good friend." She went on to study for a teaching certificate in Limerick and then taught for a number of years in Dublin at a VEC.

Once she made the leap to being a full-time sculptor, she began to travel extensively, attending symposia and workshops in numerous locations, including various parts of Ireland, Scotland, Austria, Germany, Sweden, the US, India and Japan. Symposia may sound glamorous but they are usually fairly basic in terms of budgets and facilities, and they offer little tangible return for the artist, but they do provide an opportunity to meet colleagues and to make work.

Those who know her well describe MacDonagh, with considerable justification, as being idealistic and non-materialistic.

She's certainly not lazy. Over the years, besides works made at symposia, she has completed many public commissions - the milk churns on the Cork-Mallow road, for example, and the forms based on Pot Stills in Tullamore, or the great limestone Buoy that looks out over the water at Carrigtwohill. At a land art symposium at Lough Boora in Co Offaly in 2002 she made a huge pyramid of stones recovered from the local bog workings. She says that many projects were made possible by virtue of the generosity of individuals. "Philip Harding, for example, at Technical Stonework, has been incredibly supportive over the years."

The Icosahedron series came about through a chance encounter with Sunil Agarwal of Sangreeta Granites in Bangalore. MacDonagh had met him previously when she attended a sculpture symposium in India. Now he asked her what she was doing. She said she wasn't currently working on any commissions. He had just purchased a new, state-of-the-art stone cutting machine in Italy and was keen to test its capabilities. If she would like to, he said, she could come to India and have access to it when it wasn't otherwise engaged. She jumped at the chance.

"He explained that the machine could cut a piece of stone along three axes. That's what made it possible to cut the forms of the icosahedrons." Not that they were a straightforward proposition, even so. "Indians are brilliant mathematicians, and Sunil's CAD engineer spent a fair bit of time working out the necessary computer programme for the machine."

She spent three months at Sangreeta Granites. "Bangalore is extraordinary. It's India's IT centre, yet you can move in a minute from high-tech industries to encounter ox-drawn carts. I'm wary about using the word spiritual, but you're aware in India that the past is always there with the present. It's as though you always have this context of hundreds of years of history. It gives you a great sense of perspective. I loved working in the stoneyard. It's packed with people, much more so than here, where everything is mechanised. And you know, Sunil wouldn't take any money. He just said that if I sell some of the pieces at some stage we can talk about money. He was extraordinarily generous."

From the machine, the faceted stones were carved and polished. Polishing is an intensive, sustained process that gives the granite a vitreous smoothness. Her largest icosahedron so far, Red Giant, weighs 1,633kg (1.8 tons) and is well over a metre in diameter. "I'd like to make even larger versions. It would be feasible in India, but not here." Even though not as dramatically substantial, the smaller icosahedrons have a tremendously dense physical presence, and they are irresistibly tactile.

They are so not only because of their fine, glassy sheen but because the rhythmic geometric form brings out the inherent qualities of the stone. The distinctive colours and intricate, fluid patterns of its mineral composition complement the harmonious geometry of the containing shape. MacDonagh rhapsodises about the granite, and you can see why. She has fashioned icosahedrons in a dazzling range of stones, including the ethereal Himalayan Blue, Cat's Eye with its lens-shaped clusters, the highly prized Imperial Black and the sumptuous Shiva's Gold.

"I wanted the exhibition to take the form of an installation, rather than just being a collection of pieces. So the shape is a constant. There is a basic division in material as well as form." Apart from one fibreglass work, Petrified Forest, pieces are either granite or limestone, usually Indian granite and Irish limestone. The pieces in limestone are large-scale composites. Lunar Chessboard, for example, takes the form of a grid of square-cut stones within a ring shape.

While the edges are machine-cut and smooth, the surfaces are rugged and fluid looking. "It's what the quarrymen call roughback," MacDonagh explains. "It's the natural face of the stone and it's not generally used, though I think it could be, because it's quite beautiful in itself." Scattered around the chessboard are heavy limestone spheres.

Looking around, she reflects on her own work. "Everything here is the result of slow, methodical process. If you want quick results, working in stone is definitely not for you. And an awful lot of the labour could be described as tedious in that you're just doing the same thing over and over." Nonetheless, there is more to it, she admits. "There is a meditative, therapeutic quality to it. You enter into a certain state. Your mind becomes free. You think about everything."

From Another Constellation, Eileen MacDonagh, Model Arts and Niland Gallery, The Mall, Sligo until May 28