The mature woodlands and lakes of this demesne provide the perfect setting for this turreted, battlemented and machicolated castle of gleaming silver-grey ashlar, built for the Grogan-Morgan family between 1810 and 1855 and incorporating part of a more ancient castle.
The property was presented as a gift to the Irish Nation in 1945 and was later occupied by the Department of Agriculture who established an agricultural institute here and undertook to maintain but not to alter the ornamental grounds.
The Kilkenny architect Daniel Robertson is generally believed to have laid out and planted much of the grounds in the 1830s. This would have included the digging of the five-acre lake opposite the castle with Gothic towers rising from its waters and a terrace lined with statues on the opposite bank. Many fine trees and shrubs grow in the vicinity of the castle, including two lovely examples of Cryptomeria. He was also responsible for some of the building work on the castle.
japonica ‘Elegans’, several very fine redwoods (Sequoia sempervirens), a huge Rhododendron arboreum and some of the oldest and largest specimens of Monterey cypress (Cupressus macrocarpa) in Ireland. The variety of mixed planting around the lake, which includes noble firs, Japanese cedars, Atlantic blue cedars, copper beeches, golden Lawson cypresses and holm oaks, provides a very satisfying range of colour through much of the year. In the area to the west of the castle lake, visitors will pass through a woodland garden created around the ruined medieval castle of Rathlannon.
Here the exotic foliage of a Magnolia wilsonii from China borders a large, elegant dogwood (Cornus kousa) from Japan and a Japanese snowball (Viburnum plicatum) with tiered spreading branches. Nearby lies a two-acre lake dug in the 1860s, while in the area to the north is a four-acre walled garden built between 1844 and 1851 and rehabilitated by the Department of Agriculture.
This is entered through the Devil’s Gate, an arched gateway with gargoyles that leads onto a very long gravel path lined with flower borders and backed by clipped hedges. To the tight across mowed lawns a long hothouse shelters a colourful display of plants through out the year. Steps lead to the Upper Garden, now largely devoted to shrub propagation, and the old melon yard. Here no one will fail to admire a tender dwarf Japanese maple planted in the 1880s and a range of azaleas, magnolias and hibiscus.
Other attractions at Johnstown include a cemetery with very fine wrought-iron gates made in Italy, the site of the sunken Italian Garden close to the car park, and the lower lake, dug in the 1850s and covering some fourteen acres. All three lakes in the demesne provide a home for a wide range of waterfowl – mute swans, moorhens, coots, little grebes, herons and a recently introduced flock of mallards – all of which help to control the waterweeds. The attractive early nineteenth century farm buildings to the north of the lower lake house the Irish Agricultural Museum where a variety of old horticultural implements are on display.