Terryglass Castle, also called the Old Court, is a fortified house and a National Monument in County Tipperary, Ireland.

[1] Terryglass Castle is located in the northeast corner of Lough Derg.

An early Christian monastery was established here in the 6th century by Columba, who died around AD 552.

[citation needed] Terryglass Castle is listed as having contributed to taxation of the diocese in 1302-1307 and is mentioned as having a covered chancel in the Royal Visitation of 1615, although by the Civil Survey of 1654–56 only a churchyard is mentioned.

The castle was to change hands many times passing to Nicholas Dunheaued in 1275–76, Theobald le Botiller, 2nd Chief Butler of Ireland in 1289, William Marshall in 1290, Richard Óg de Burgh, 2nd Earl of Ulster in 1323 and James Butler in 1589.