Press the right coded electronic buttons at the entrance to Castlegar House and the great iron gates slowly swing open on to an avenue that winds its way through an endless expanse of parkland. After meandering for half a mile past ancient trees, the avenue twists its way up a gentle slope to where the house sits on a height overlooking the encircling lawns and woods.

It has stood there since 1803, exalting testimony to the taste and distinction of late Georgian architecture. Castlegar is hidden away among 50 acres of gardens, parkland, woods and pasture outside the village of Ahascragh, in east Galway.

It is for sale by private treaty through Charles Smith, of Gunne's country homes division, who is quoting a guideline price of £1.5 million.

Originally, the estate was the home of the Mahons, gentry stock whose descendants linked with the Guinness family to form a land agency that eventually evolved into the Guinness Mahon merchant bank.

Sir Ross Mahon commissioned architect Richard Morrison to plan alterations to a rambling old house that existed there previously. Rather than remodelling it, Morrison designed an entirely new building which took several years to complete.

Since 1992, Castlegar has been owned by a Frenchman with a passion for restoring old houses to their original splendour and who has spent hundreds of thousands on refurbishing it. He is now selling it as he is unable to spend enough time there because of commitments in Paris, the US and Canada.

He is leaving one of the finest Georgian country homes in Ireland, restored with consummate care to the pristine state of its early days. The marvel of the restoration work lies in the fact that while it has uncovered the innate beauty of the house as it was first conceived, it also has added all the appurtenances of modern living.

Castlegar has been described as the grandest of Morrison's "villas", the word villa being used in its original meaning of a country residence.

The house combines resplendent reception rooms with exceptionally comfortable family accommodation in an ambience of relaxed old-fashioned elegance.

In addition to the staff accommodation, there are six bedrooms, each with a fireplace and its own bathroom, and all providing views across the rolling plains of east Galway.

Oddly, the house has two entrances, one on the north side, the other on the south. The south entrance, no longer used as such, opens into an oval hall with a magnificent ceiling adorned with classic floral friezes, a white marble mantelpiece, and columns flanking recessed doors that lead to the drawingroom on one side and a morningroom on the other.

Two other doors open on to the top-lit central stair hall, an elegant space where the Portland stone staircase has a simple, wrought iron balustrade and ascends to an imposing domed landing.

The oval hall, the huge drawingroom and the diningroom were radically decorated at the turn of the century with commendable taste and the present owner has attentively preserved and enhanced the adornments. The drawingroom, which has a polished, pitch pine floor, is graced by a striking period mantelpiece with an Adam-style grate.

Classic Victorian-style predominates in the diningroom where there's a high fretted ceiling, a carved oak mantelpiece and heavy oak shutters. A spacious billiards room-cumlibrary, with a large, hand-crafted oak mantelpiece, and a beautifully appointed study are other impressive features of Castlegar.

In addition to the six bedrooms on the first floor, there is another spacious drawingroom looking across a fountain and lawns to the south. The staff quarters are located on the second floor. There are a further two bedroomshere as well as a kitchen, sittingroom and bathroom. Walled gardens, a stable complex and a hard surface tennis court are spread out over several acres close to the house.

The outbuildings include a beautiful lofted cut-stone coach-house, along with four garages and three stables, plus a stable yard that has seven loose boxes, a tack room and a further spread of farm buildings.

Beneath the house is a vaulted basement, dry and airy, with six rooms, a boiler space and a wine cellar.