Although many books devoted to Irish country houses have appeared in recent years, relatively little has been published about their contents. This is not least because few of the buildings are still owned by the families for which they were built. Over the past century, an enormous number of houses were destroyed or allowed to fall into ruin. Those that survived were often sold and their contents dispersed; the documents that might have recorded when and how material was acquired were just as frequently lost.

The result is that very little is known about how large Irish houses were decorated and furnished in the 18th and 19th centuries. Examination of old auction catalogues, especially those dating from the 1920s to the 1950s, when so many house sales took place here, indicate that the quality of pictures and furniture was equal to that elsewhere in Europe.

Yet the only concerted effort to present information on a private art collection has been an exhibition devoted to the Milltowns of Russborough, organised by the National Gallery of Ireland four years ago.

Today, a show with a similar theme opens in London. Clerics And Connoisseurs: An Irish Art Collection Through Three Centuries examines in meticulous detail the cultural history and interests of the Cobbe family, which has lived in Newbridge House, near the north Co Dublin town of Donabate, since it was built, between 1747 and 1750. The Cobbes continue to be closely associated with Newbridge, even though in the mid-1980s the property was acquired by the local authority, now Fingal County Council.

Most importantly, when responsibility for the maintenance of Newbridge changed hands, the family agreed to leave on loan the house's pictures and furniture. While not the grandest or greatest of 18th-century Irish properties, it is one of the most significant, as it allows visitors to see a series of rooms that have scarcely altered for more than 200 years.

Newbridge was constructed for the Rev Charles Cobbe, who moved to Ireland from England in 1717, when his godfather, the second Duke of Bolton, became Lord Lieutenant. Although the duke left soon enough, Charles Cobbe remained, and he enjoyed a highly successful clerical career, rising to be Archbishop of Dublin in 1743; it seems he expected to become Archbishop of Armagh and was disappointed when the position went to another man.

Newbridge was built around the time of this setback. Charles Cobbe had bought the land on which it stands in 1736, planning to build what would have been one of Ireland's most palatial Palladian houses. His failure to secure the Armagh primacy led him to settle for the more modest, but still very handsome, house that stands there today.

Newbridge was inherited by the archbishop's eldest son, Thomas Cobbe, who in the 1760s added to the house a large drawing room with an elaborate rococo stuccoed ceiling. Doubling as a picture gallery, the space was designed to accommodate his paintings. Most of them remain where they were first hung, although the family sold three of the most valuable in the 1830s, to pay for the construction of estate cottages.

Thomas Cobbe was advised in his purchases by the local vicar, Matthew Pilkington, who was described by Jonathan Swift as "the falsest rogue in either kingdom" and who caused a scandal by divorcing his wife on the grounds of adultery. He better deserves to be remembered as the well-informed author of The Gentleman's And Connoisseur's Dictionary Of Painters. Published in 1770 and reprinted in numerous editions over the next 100 years, the book was the first guide to approximately 1,400 European artists as well as notable art collections of the period. Under such circumstances, it is not surprising that the pictures assembled by Thomas Cobbe were of such a high standard.

The exhibition at Kenwood House, on the edge of Hampstead Heath, has been organised by another of the archbishop's art-loving descendants, Alec Cobbe, who grew up at Newbridge in the 1940s and 1950s, when the house still had no electricity and was lit by oil lamps. The term polymath tends to be applied with excessive ease, but in Cobbe's case its employment is justified. A musician, painter, picture restorer, art historian, interior decorator, glass engraver and collector, he intended to become a doctor, studying medicine at Oxford until he was distracted by his other interests.

Today, he lives at Hatchlands Park, a mid-18th century house in Surrey, decorated by Robert Adam and now run by the National Trust, which has been filled with the Cobbe Collection, a group of 43 musical instruments that includes pianos used by Chopin, Mahler and Elgar, one signed by Johann Christian Bach (son of Johann Sebastian) and the keyboard on which Bizet composed Carmen. Cobbe has played on all of them and two years ago released a double CD.

Despite musical interest and ability, his principal work for the past two decades has been in the field of interior design, for public museums and private houses. Among the former have been schemes at Windsor Castle and the Palace of Holyroodhouse, in Edinburgh; among the latter are homes in Britain and Ireland. Cobbe's upbringing at Newbridge has left an indelible mark on his approach to the ordering of rooms, especially the way in which pictures are displayed.

Traditionally, works of art in private houses were arranged with an eye both to their individual merit and to their collective impact, with paintings hung two or three deep. The advent of public museums and galleries, in the late 19th century, led to a change of practice, with work thinned out and shown in isolation. The red drawing room at Newbridge has always been a perfect example of the older practice, and it is this approach to picture hanging that Cobbe advocates.

Last year, he was invited to advise on the reordering of the dining room at Kenwood House, a property that has had strong Irish connections since 1922, when it was bought and presented to the British public by Edward Guinness, first Earl of Iveagh; on his death, six years later, he bequeathed Kenwood a superb collection of 17th- and 18th-century old masters. The paintings are still on display on the house's ground floor; five rooms on the storey above are devoted to the Cobbe exhibition for the next three months.

The show is a mixture of paintings from the collection assembled at Newbridge during the 18th century, including a Hobbema landscape now owned by the National Gallery of Art in Washington, DC, plus pictures that Cobbe has bought over the past three decades or so. In many respects, this second group is even more remarkable than that acquired by his forebears. Cobbe's astute eye means his collection now includes work by Il Guercino, Titian and Poussin. Their ownership has been vested in a trust, so any future loss need not be feared.

The demands of Cobbe's work means he regularly comes back to the Republic at the request of clients, but opportunities to stay at Newbridge House are not as frequent as he would wish. He is full of praise for Fingal County Council's programme of restoration work on the house and its demesne, which includes the overhaul of a walled garden.

But even when he is not here, his research into Cobbe family history continues; earlier this month, he co-wrote a feature in Country Life demonstrating that Newbridge had been designed by James Gibbs, making it the only known work in Ireland by this English architect. That the building and its contents have survived almost intact is remarkable, given the more customary misfortunes of Irish country houses. The Kenwood exhibition acts as a reminder that while a fine example of family connoisseurship has been saved, within recent memory far too many others were needlessly lost.

Clerics And Connoisseurs: An Irish Art Collection Through Three Centuries is at Kenwood House (00-44-20-83481286), Hampstead Lane, London until January 27th 2002