It is located at Bryansford, near the town of Newcastle in the Mourne and Slieve Croob Area of Outstanding Natural Beauty.

The river is a spawning ground for salmon and trout and is an Area of Special Scientific Interest due to its geology, flora and fauna.

The extended Magennis families controlled most of the land in the south of County Down by the 15th century, including the area where Tollymore is located.

On 22 February 1611, Tollymore (officially referred to as Ballytollymore) was included in seven and a half townlands which were surrendered to the English crown in return for a formal freehold in the name of Brian McHugh McAghorley Magennis.

This was likely because James Hamilton had raised a regiment in support of William of Orange and was providing the Williamites with supplies.

[9] Tollymore features many follies whose design were influenced by Thomas Wright of Durham, a friend of James Hamilton, 1st Earl of Clanbrassil.

[10] At the entrance and exit to the park there are Gothic gate arches built by James Hamilton, 2nd Earl of Clanbrassil (Viscount Limerick).

[15] There are numerous artificial and natural features along the Shimna River, including bridges, grottos and caves.

The Hermitage is a small stone shelter designed by James Hamilton (2nd Earl of Clanbrassil) in the 1770s in memory to his friend the John Montagu, Marquess of Monthermer.

A rise in grey squirrel numbers, first seen in the park approximately 10 years previously, was noted at the same time.

Since then The Tollymore Red Squirrel Group, consisting of volunteers, forest service staff and members of the Mourne Heritage Trust, have been working to preserve the species within the forest by providing additional food and attempting to reduce the numbers of grey squirrels.

[26][28] In 2007 a breeding pair of great spotted woodpeckers, which had not bred in Ireland for centuries, were recorded in Tollymore for the first time.

[13] A population of approximately 20-30 pairs of feral Mandarin ducks have been present on the Shimna river and on a pond within the park since 1978.

[30][31] Pine martens, which are one of the rarest native mammal species in Ireland,[32] are frequently spotted in the park by volunteers monitoring the squirrels.

It contains trees from many different parts of the world, including the remains of a giant redwood which was struck by lightning.

[13] The slightly milder climate in the area, due to its proximity to the coast, provided more favourable growing conditions than further inland for the more exotic trees in the arboretum.

Other trees present include field maple (Acer campestre), Himalayan cedars (Cedrus deodara), coast Douglas fir (Pseudotsuga menziesii var.

[37][38] Shrub species found in the park include Rhododendron, elderberry, blackberry, laurel and ivy.

Lawn areas contain bent (various Agrostis species) buttercups, clover, dandelions, meadow grass and perennial reygrass (Lolium perenne).

[30] In 2012, 50 hectares of the invasive Rhododendron ponticum was removed from forests in the Mourne area, including Tollymore.

The project was carried out to improve the quality of the woodland and prevent the spread of Phytophthora ramorum, which the species is known to host.

[39] Algae (Nitella flexilis) and ferns (Hymenophyllum wilsonii, Phegopteris connectilis, Polystichum aculeatum and Dryopteris aemula) are also common.