Dysart Castle, Medieval castle ruins near Thomastown, Ireland

Dysart Castle is a medieval fortress beside the River Nore near Thomastown, featuring substantial stone walls with large windows visible on its upper levels. A church stands immediately adjacent to the structure, forming part of the same site.

Dysart's lands and structures were granted to the Augustinian Priory of Kells by its founder Geoffrey fitz Robert in 1193, and the charter names the site Dysart of St. Mo-Colm-óg, or Colman. The church and castle stayed with the priory until its suppression in 1540. Archaeologist Ben Murtaugh's excavation, reported in 2009, identified four periods of activity, from an Early Medieval monastic site to a post-medieval dwelling house built south of the tower house.

The site is connected to the Berkeley family, who made their home here and produced the philosopher George Berkeley. His name remains linked to institutions in California and Trinity College Dublin.

The property is privately owned and the castle is in urgent need of restoration, with cracks in its walls prompting a 2003 appeal over risk of imminent collapse. The ruins stand low along a sharp bend of the River Nore south of Thomastown, so approach with care and view the masonry from a safe distance.

Excavations led by archaeologist Ben Murtaugh in the 2000s, with funding from the Royal Irish Academy and a summary report in 2009, identified four periods of activity at the site, from an Early Medieval monastic settlement associated with St. Colman to a post-medieval dwelling. Finds included two cross slabs and Romanesque architectural fragments.