It took a few moments to summon the courage to knock on the door of one of the most fascinating buildings in Dublin. Peter Pearson opened the door and invited me into No 2, Palace Street, known to many as the Sick and Indignant Roomkeepers' House, because of the writing on its facade. Of course, it actually refers to the Sick and Indigent [poor] Roomkeepers' Society, which still exists and carries on its business in more modern surroundings in Leeson Street. The Pearson family moved into the 18th-century house in 1993, a move which Pearson describes as "impulsive". "We were going to move to the country, and then this building came up." He says he has always been attracted to living in period houses, as well as feeling he has some sort of duty to do so: "If you're going to encourage people to save old buildings, you should be prepared to do so yourself."

Peter Pearson is first and foremost a painter, but for many years he has had a high profile as a conservation campaigner. Involved in the Drimnagh Castle conservation project since 1986, he is a member of the Heritage Council and a founder-member of Dublin Civic Trust. Most recently his name is on people's lips because the fashionable coffee-table accessory for Dublin southsiders at the moment is his book, Between the Mountains and the Sea: Dun Laoghaire- Rathdown County. A record of buildings and monuments past and present in Dun LaoghaireRathdown, its 370 or so pages contain more than 700 photographs, many of them taken by Pearson himself, as well as entries on any building of interest in the borough built before 1900.

One of the most fascinating, although melancholy, aspects of the book is that so many of the lovely Georgian and Victorian houses recorded in it have disappeared. Most famous of the demolished buildings is probably Frascati House, the ruined shell of which was pulled down in 1983 to make way for a shopping centre. Among the many interesting things in the Sick and Indigent Roomkeepers' House is a collection of scraps of plasterwork salvaged from the rubble of Frascati and other houses.

Although much of the destruction took place in the 1960s and 1970s, beautiful houses are still being demolished. Redesdale in Stillorgan, which is described in the book as "one of the last 18th-century houses in the whole area . . . of historical significance," was pulled down in August 1998, disappointing Pearson who had planned to revisit its interior. "In my naivete, I thought the worst excesses of that were over; I thought I'd go back and photograph it next time - and it was gone." He hopes forthcoming planning legislation, which will cover interiors and fittings, as well as the facades of listed buildings will protect the remaining buildings which are part of the area's, and the country's, heritage.

"We've just been so bad about preserving these things, and then added to that is the intense pressure on south Co Dublin because it's such a rich area and has the highest property prices in Ireland."

Although the stately homes of the borough make up most of the houses covered in his book, cottages such as those which used to stand along the road in Sallynoggin are also represented. "It's just as important to see the vernacular preserved as the bigger ones. There's one thatched cottage in Loughlinstown, and I'm open to correction, but I think it's the only one left in the area." One-roomed quarrymen's cottages at Kilgobbin are also illustrated.

Unlike the apocalyptic style of many conservationists, who would have you believe that we will soon have only modern little boxes and garage forecourts, Pearson speaks in a tone of optimistic realism. "The area is very extensive: it includes from St Columba's College right at the back of Rathfarnham, and Marlay House, down to the sea at Merrion Gates, and right around to Bray and then right back into the mountains as far as Glencullen. If you go up to the mountains and actually look down on it, it's amazing how much of it is preserved intact at the moment. Even today, at the end of the 20th century, it's still very diverse."

He points out that the 19th-century panoramic view of the Dublin mountains which is used as an end-paper shows a wild mountain landscape not much altered today. "There are other interesting areas like Glencullen and Glencullen House, which is very beautiful. When the Ordnance Survey made their first survey in that village, they discovered a butter well, where the people washed their butter-making implements to avoid the butter turning sour. It's still called the butter well."

Only one 20th-century building, Michael Scott's house in Sandycove, is included. When I ask whether this was a value judgment, he says: "I didn't really discuss the 20th century much, or 20th-century architecture, for the simple reason that it's the job of people to come in the next century to assess the heritage of our century. I don't think it's our job to assess recent building. The older buildings have been disappearing more rapidly, and I do feel it's more urgent to record them."

That is, after all, the purpose of the book: to create a record of Dun Laoghaire-Rathdown borough; its buildings and history. Pearson sees this as a social necessity - the popularity of the book, (which is already out of print, though it should be in the shops again soon), is an indicator of how interested people are in the history of their area.

He breaks off here to tell me about how a Texan called on him once in Palace Street, with an old diary telling how one of the American's forebears had worked in the building as a tailor.

"We were able to find the documents showing when his ancestor had worked here. That's nice when you can still find connections with descendants still alive. I think that's what's lost when so much of the city is destroyed, and the county too. Just wiped out - and all connection with the past is gone. There's a whole cutting off of identity and connection. "That's part of what I'm interested in in the book, really, trying to re-establish those things. I've had an awful lot of letters from people who lived in these places, and are trying to research their own family history."

He sees this interest as one of the most important factors in conservation. "In fairness, most of the owners of old properties do look after them very well, because they own them and take an interest. In general it has to be said that the people who live in an old building look after it and love it. What does happen is, if a big institution is sold, it tends to be a builder who buys it, and there's a sacrifice then."

However, not all is doom and gloom, even with regard to houses no longer in private ownership. Pearson lists several examples of "good stories," including Marlay House, "a beautiful Georgian house with fantastic interiors, built by the LaTouches". It is being restored by Dun Laoghaire-Rathdown County Council, and will eventually be open to the public.

"Then there's Cabinteely House, which is also owned by Dun Laoghaire-Rathdown, and that's going to be opened with a vintage car collection. Again, very fine interiors, and a very impressive place.

"The last one is Airfield House in Dundrum, which is very famous. Airfield was left by Laetitia and Naomi Overend in trust to the State, and it's going to be an organic farm open to the public. There is a tradition of dairying in the area, especially with herds of goats. That's where the name Goatstown comes from." We search the book for a 1788 engraving of a view of Dublin Bay from Kilmacud, showing a goatherd and his goats in the foreground.

Among the projects Pearson would like to see under way in the borough where he grew up, is a walk-way all along the coast from Merrion Gates to Dun Laoghaire. "That's the sort of thing that could be developed at very little cost, and it would be hugely used. To mem that would be a worthwhile millennium project."

Between The Mountains and the Sea, by Peter Pearson, is published by O'Brien Press, price £25