Donamon Castle, Medieval fortress in County Roscommon, Ireland.

Donamon Castle, properly Dunamon Castle (Irish: Dún Iomáin), stands on raised ground above the River Suck near Williamstown in County Roscommon. The name means "fort of Iomán", and a fortification has occupied the site since early times. The castle is one of the oldest continuously inhabited buildings in Ireland, and the Divine Word Missionaries, who bought it in 1939, added several new buildings around it to form a training campus.

Dún Iomáin is first recorded in the Annals of the Four Masters for 1154, seat of the Ó Fionnachta chiefs of Clann Chonnmhaigh. Adam de Staunton fortified it in 1232, but the O'Connors retook and demolished it the next year. Rebuilt and held by de Oddingseles in 1294, it passed to the de Birminghams, was destroyed again, and in 1307 came to the MacDavid Burkes, who held it for 300 years.

The Missionaries of the Divine Word took over the castle in 1939 and transformed it into a place of education and spiritual work. They changed its character from a feudal fortress into a center for religious training.

Donamon Castle is a working missionary residence and training campus, not a visitor attraction, so there are no tours or opening hours. The grounds lie near Williamstown, County Roscommon, and the castle itself is private; the River Suck and the surrounding raised ground can be seen from public roads nearby.

In late 1932 an IRA unit under Seán McCool and Mick Price seized Donamon Castle to set up a training camp, an episode rarely mentioned beside the castle's medieval history. The Divine Word Missionaries bought the property from the Irish Land Commission in 1939. From 2019 the former training campus was managed by the Irish Wheelchair Association as a National Holiday Centre, and in 2022 Roscommon County Council reopened it to house Ukrainian refugees, with help from Irish Aid and the Roscommon Lions Club.