Gardens and the park at this ancient castle were started around 1740 and some of the original trees still survive onsite. The Demesne extends to over 100 acres and is open to the public every day of the year. It contains formal gardens, terraces, gardens along the banks of the river and a particularly important arboretum. Most of the important plants in the collection are of Chinese origin and a number are certainly the finest specimens known in Ireland.

During the 1840's, the third Earl of Rosse designed and built the mirrors, tube and mountings for a 72 inch reflecting telescope which was the largest in the world at that time. It is still located on the grounds.