The arboretum was established by the fifth Earl of Annesley (1831-1909) who inherited Castlewellan demesne from his brother in 1874. This lush allotment was originally confined to the Walled Garden area, a short distance north east of a huge baronial castle that had been built between 1856 and 1859 with glorious prospects of the park and the Mourne Mountains. In 1967 the demesne together with the arboretum were purchased from the late Gerald Annesley by the Forest Service and two years later opened as a Forest Park. The arboretum, which originally covered no more than fifteen acres, has since been considerably expanded and now covers more than one hundred.

The hub of the collection is the Walled Garden, now called the Annesley Garden. It is divided by a wall into two portions: the Upper Garden, originally built in 1740 to cultivate kitchen produce, and to the south east the larger Lower Garden - laid out in the 1850s as a pleasure ground with terraces, a fountain and ornamen tal trees. The two areas are united by a formal axis running down through the centre, which in the Upper Garden is lined with herbaceous borders backed with clipped yew hedges.

The main entrance brings the visitor into the Lower Garden, passing the old gardener's house on the left. The path here has some striking Chilean fire-bushes (Embothrium coccineum 'Longifolium'), rhododendrons and maples. The path to the right passes a large Chamaecyparis pisifera 'Squarrosa' first introduced from Japan in 1862 and now a popular cultivar. As in many old arboreta, the family Chamaecyparis is well-represented and there are over forty varieties here. Further down the path and flanking a flight of steps stand the oldest trees in the garden - a pair of Wellingtonia (Sequoiadendron giganteum) planted in 1856 no less than three years after the species was first introduced from Sierra Nevada. Close by an extremely large Monterey cypress, Cupressus macrocarpa 'Lutea', is hard to miss; this is the mother tree of Cupressus x Cupressocyparis leylandii 'Castlewellan Gold', one of the best-known but perhaps least endearing trees in this arboretum. It was discovered in 1962 by the then head gardener John Keown after extracting and sowing seeds from a cone-laden branch. Millions of these trees now grace (or disgrace) the gardens of the world, not least in Ireland where they are commonly associated with brash bungalows.

In the same area along this path stands a slow growing Cordyline indivisa 'Vera' - one of a large number of notable plants at Castlewellan that comes from New Zealand. Amongst others is the genus Podocarpus of which there are fifteen species; visitors will also discover good specimens of the evergreen Carpodetus serratus, the aromatic Pseudowintera colorata, the unusual kawaka (Libocedrus plumosa) and an exceptionally beautiful Pseudopanax laetus.

Many fine trees grace the area around the heron fountain where the planting becomes even more profuse - here, for example, a large specimen of Picea smithiana, planted in 1868 and now seventy-five-feet tall, makes its home. There are forty-two members of this genus in the arboretum. Close by stands a lovely Myrtus apiculata and, also from Chile, a remarkable Pilgerodendron uviferum. Throughout the arboretum there are generally some thirty species, varieties and cultivars of the Juniper genus, including a Juniperus recurva that is fifty-feet tall.

Beautiful views of the Mournes can be obtained from Castlewellan

The silver fir is also well represented with forty-five members, the oldest an Algerian fir, Abies numidica, planted in 1886. Among the most dignified plants from the southern hemisphere is a fine Tasmanian waratah, Telopea truncata, while along the north-east wall leading up to the greenhouses stretches a magnificent avenue of late summer flowering eucryphia.

The Edwardian greenhouses in the corner of the Lower Garden contain quite a variety of fuchsia, passiflora and tibouchina, as well as specimens of Lapageria rosea and Wattakaka sinensis. Like jewels in a box, one greenhouse encases a small collection of free-flying tropical birds; in the two adjacent end houses lush collections of ferns and phormiums grow.

Immediately north of the Annesley Garden lies an extensive collection of spring-flowering trees, mainly prunus and malus planted between 1972 and 1974. A prodiguous grouping of dwarf conifers also thrives, sown in 1976, the trees occupy four long, curved beds. In the shade of the beech trees to the north is a five-acre rhododendron garden which also includes camellias and large numbers of deciduous azaleas. Two duck ponds on the fringe of this area display many outstanding trees within their vicinity, including a very tall red silver fir (Abies amabilis).