Gardening: Temples of mirth and jollity: In Ireland follies have, ironically enough, served some practical purpose. Anna Pavord takes a look at a new book that traces the tradition of 'monuments to mood'

Folly-building is not an activity that you can imagine the Department of Social Security recommending as a way to alleviate unemployment or poverty. It was, however, much in vogue in 18th-century Ireland, where inscriptions on many towers, columns, obelisks, sham ruins, mausoleums and eye-catchers record that they were put up to 'give employment to the poor of the district'.

Conolly's Folly at Castletown, Co Kildare, is one of the most famous: an obelisk raised on a series of arches arranged like an acrobats' pyramid, each row resting on the shoulders of the one below. It was commissioned by Katherine Connolly to commemorate her husband, speaker of the Irish House of Commons in the 1720s, and to provide famine relief.

The Connollys were folly fanatics, as James Howley makes clear in his new book, The Follies and Garden Buildings of Ireland (Yale, pounds 35). Famine relief gave them justification; but you also need to be temperamentally inclined to folly-building, and to have the spare cash. The Connollys scored on both counts, and in a staggeringly short space of time commissioned folly pigeon-houses, a superb folly barn - built as an enormous conical tower, with four bays projecting at right angles and a stone stair winding round the outside of the cone - a temple by the Liffey (which runs through the grounds of Castletown), sphinx gate piers and an extraordinary Gothic lodge, bristling with pinnacles. All have survived.

Mr Howley's book, well illustrated with black and white photographs and plans, takes a wide view of the word 'folly'. So did Lord Clark of Civilisation fame, who defined them no more precisely than as 'monuments to mood'.

They are also monuments to megalomania and madness. Folly builders include a larger than average proportion of people who insisted on being buried standing, or upside down, or on horseback, the better to survive the Day of Judgement. Only Ireland, though, could produce a squire such as Robert Watson of Larch Hill, Co Meath, master of the local hunt. After a lifetime happily engaged in folly building - the Cockle Tower (encrusted with shell work), an ornamental dairy, a miniature mock fort on an island in his lake, two rustic gazebos and a circular temple - he decided he was going to be reincarnated as a fox. His final folly was his mausoleum, the Fox's Earth, an artificial chamber set into an earth mound and topped by a circular temple of rough stone. Two fox-sized tunnels lead to its interior . . .

Mr Howley's book is arranged by type of folly rather than according to date. This has some disadvantages. It makes it difficult, for instance, to follow the engaging careers of folly builders such as Watson, whose work, under this arrangement, is scattered over five different sections. It is a moot point, too, where some follies fit best. They are not by nature easy to categorise. The Luttrellstown rustic arch of Chapter 3 is not so different in style and execution from the rustic bridge at Danesmote House that appears in Chapter 11.

Eccentricity, cost or conspicuous uselessness are the hallmarks of most good follies and they were thickest on the ground in the first half of the 18th century, before style gurus such as Richard Payne Knight decided that follies did not form part of their ideal of the picturesque. Fortunately, landowners were sensible enough to make up their own minds on this subject and follies continued to be built, though in a less classical mode, throughout the 19th century.

Unfortunately, they have not always been well maintained. Even a sham ruin needs attention and Mr Howley's book emphasises the thin line that exists between a picturesque relic and a heap of rubble. In this country, the Landmark Trust has specialised in restoring and finding a use for English follies, by renting them out as holiday escapes. A similar society has recently been formed in Ireland and could provide the means of rescuing the kind of folly that kept at least a nodding acquaintance with function.

The Temple of the Winds at Mount Stewart, Co Down, is a perfectly preserved example of this type of folly, a handsome octagonal building that seems to have been used as a banqueting house during the time of the third Marquess of Londonderry. He replied robustly to the suggestion that his building should be converted into a mausoleum to commemorate Lord Castlereagh. 'I have no Taste for Turning a Temple built for Mirth and Jollity into a Sepulchre - The place is solely appropriate for a Junketting Retreat in the Grounds.'

Junketting retreats do not come high on the list of priorities in modern gardens, but James Howley is able to bring his story up to date with work commissioned by two prominent Irish landowners. At Glin, Co Limerick, a rustic arbour has recently been added to the follies (bathing lodge, eye-catcher, hermitage) built by the Knight of Glin's ancestors. At Leixlip Castle, Co Kildare, Desmond Guinness has gone Gothic with a new gate lodge and a brilliant greenhouse with brick and cobble walls, stone ball finials and tall, pointed windows. I want one, now.

(Photographs omitted)