Following these Williamite Wars of the 1690s, a period of peace and prosperity pervaded Ireland. The new owner of Thomastown Castle, Thomas Tenison, took advantage of this prosperity and remodelled the house in 1754.

The castle changed hands and came under the ownership of the O'Reilly's towards the end of the 1700's. In the mid 1860's, following the Act of Union of 1801 and the Great Famine of 1847-1850, Ireland entered another evolutionary era. Following his service with the Papal Brigade in Italy in the 1860s, Myles O'Reilly returned to Thomastown Castle, and continued the ambitious redevelopment programme of the house and gardens that he had started in 1858.

Renaming the estate Knock Abbey, Myles O'Reilly commissioned architect William Caldbeck to add a new house. This required substantial alterations to the original tower house including fitting Gothic Revival windows and rebuilding all the battlements.

The addition to the east and south, including the contents of the O'Reilly family library, were lost when the Irish Republican Army broke into Knock Abbey and set fire to it in 1923. What survived was rebuilt in 1925 to W.S. Barker's design and is being given a new lease of life by the current owner, Cyril O'Brien.

The southern facade of the 1925 extension