Ashtown Castle was not discovered until 1978, as it was enclosed within the walls of a building for years. When this building was being demolished, the castle was discovered. The castle was originally owned by John Connell in 1641, who would have received a grant from the government to build the castle in order to defend himself.
In 1668, the castle was acquired by the Lord Lieutenant of Ireland on behalf of the King to extend his deer park, which is why the deer still roam the park today. In 1774, the castle was incorporated into Ashtown Lodge, and the castle disappeared from sight. The Lodge became the official residence of the Under Secretary from 1782, and it is believed that a series of tunnels ran underneath, leading to other important buildings in the park, so that people could travel without being seen.
The last resident of the lodge was Papal Nuncio, who left in 1978. It was then that the castle was discovered when the Lodge was being demolished due to dry rot. The castle was restored to its former glory, and is now part of the Phoenix Park Visitor Centre.