The spectacular ornamental grounds and gardens surrounding the 19th century castle were designed by Daniel Robertson who is also famed for the gardens at Powerscourt in Co. Wicklow.
Johnstown Castle has been home to various interesting families over the centuries.
The first owners were the Esmondes – a Norman family which settled in the county in the 1170s. They constructed the tower houses at Johnstown and Rathlannon.
During the Cromwellian period of 1650s the estate was confiscated and changed hands several times before being acquired by John Grogan in 1692.
His descendants owned Johnstown up until 1945 when Maurice Victor Lakin presented Johnstown Castle estate as a gift to the Nation.
By the 1880s Johnstown Castle was at its peak of development and comprised of a large demesne of over 1,000 acres.
It includes two castles, ornamental pleasure gardens, three lakes and working research farms.
Teagasc own the entire estate and have been carrying out pioneering agricultural-related research on site for over 50 years.
The Department of Agriculture and EPA also have offices on the estate.