The Countess of Burlington's ultra-romantic gardens at Lismore Castle in Ireland

As befits a house in which the 'father of modern chemistry' Robert Boyle was born in 1627, Lismore is a place of fusion and experimentation. The appearance of the castle today is largely the work of Sir Joseph Paxton and the 6th Duke of Devonshire, incorporating elements of Sir Walter Raleigh's original castle. And like house, like garden, as the seven acres, neatly divided into the Upper and Lower Gardens, are a glorious agglomeration of sustained horticultural expertise. The walled Upper Garden was first constructed by Richard Boyle, the 1st Earl of Cork, around 1605, while the more informal, woodland Lower Garden was created in the nineteenth century for the 6th Duke of Devonshire. Although the walls and terraces of the Upper Garden remain as they were when they were first commissioned in the seventeenth century, each subsequent generation has added to the patina, with much work done by William's parents. They were also pivotal in the introduction of sculpture to the garden.

The garden guide that each visitor receives charts the unique castings and pieces of contemporary sculpture that have been commissioned by William's parents since 1999, conjuring covetousness in an art lover as intensely as the glossary of notable plants does in a horticulturist. In the dusky setting of the Yew Avenue in the Lower Garden, a lonesome Antony Gormley figure looks out from the shadows, and a trio of David Nash oak columns punctuates a grassy clearing. The guide also lists follies with eccentric names, from Broghill's Tower in the top south-east corner to the Monkey Tower, situated 50 metres below in the Lower Garden.

The Sundial Garden was reworked in 2009, as a surprise for William's birthday, with new curved oak benches by Henry Brudenell-Bruce; Rodgersia pinnata thrives here under the ornamental cherry trees.

Andrea Jones

Lismore is possibly the oldest continually cultivated garden in Ireland, but it has a palpable spring in its step thanks to head gardener Darren Topps, who arrived from the Eden Project four years ago. Darren succeeded Chris Tull, whose son Matthew remains an integral part of the five-strong garden team. It was Matthew who helped Laura on a reworking of the Sundial Garden as a surprise for William's fortieth birthday in 2009 - a cloak-and-dagger collusion abetted by Henry Brudenell-Bruce, who made the oak circle seats.

'We wanted Lismore to be part of Ireland's wonderful gardening culture,' says Laura, who has been astounded by the momentum Darren has built up. Within a year, his team had nominated him for the Devonshire Group award - a scheme run by the Duke of Devonshire and the Chatsworth Estate to recognise the employee of the quarter. He won. 'A lot of the things we like, Darren does too, or he pretends to… Occasionally I'll mutter "no orange flowers", but anything he combines looks wonderful,' says Laura. There are a few beautifully integrated orange flowers in the main herbaceous border that is aligned with the spire of St Carthage's Cathedral in Lismore. 'The palette starts hot and graduates to creams and whites,' says Darren, who is constantly expanding the gardens' plant collection and diversity. When Laura arrived at the Great Dixter Plant Fair earlier this year - 'empty car, cash in pocket, no children' - the first person she saw, getting out of a van in the car park, was Darren.

The castellations of Lismore Castle seen from Broghill's Tower. Trained fruit trees are interspersed by topiary

Andrea Jones

The plant list starts very appropriately with a magnolia, M. hypoleuca, and ends with Cryptomeria japonica, a Japanese cedar. The latter was grown from seed given to the then Duchess of Devonshire by her sister Pamela Mitford in the Fifties, a few years before the adjacent orchard was planted - in case you wonder why this exotic beauty is rubbing shoulders with the exquisitely pruned apple trees in their froth of hand-scythed flower-rich meadow.

Salvias are threaded through the garden, and there are hot spots that take advantage of the microclimate where echiums, isoplexis, watsonias and erythrina thrive. Swathes of vegetable garden rotate with the annual sowing of Pictorial Meadows' Pastel seed mix, beautifully encased in hoops of freshly cut willow. Surplus fruit and vegetables go into the Lismore Castle vegetable box scheme, surely the chicest on the planet and a snip at €10 a month. Apothecary herbs and Mediterranean plants bask in the Upper Garden, while rare trees and shrubs, including a monumental group of Eucryphia x intermedia 'Rostrevor', flourish in the Lower Garden, planted to frame Eilis O'Connell's bronze Under and Over IV. Between the gardening team, the Lismore Castle Arts team and Laura and William, another lustrous layer of Lismore romance is being applied to the unbroken chain of creative ventures.

Lismore Castle Gardens are open daily between March and October, 10.30am-5.30pm; lismorecastlegardens.com