The southeastern tower of Roscrea Castle is sometimes called King John's Castle, and there is evidence to support the name. In 1213, John, King of England, ordered a "motam et bretagium" -- a motte and tower -- built on this site as part of his efforts to consolidate control of the Irish midlands. The land belonged to the Bishop of Killaloe, and construction was overseen by the Justiciar, Henry of London. The original wooden castle was destroyed in the late 13th century and replaced with the current stone structure. Between 1274 and 1295, more than 875 pounds was spent on works under John de Lydyard, producing a 40-metre-wide courtyard enclosed by curtain walls up to 2.5 metres thick, originally surrounded by a river to the east and a moat on the other three sides.

Roscrea Castle has no keep. Instead, the main residence is a three-storey rectangular gate building to the north, rising approximately 27 metres, originally equipped with a bascule drawbridge and portcullis. The entrance features a barrel vault ceiling, and below the gate tower lies a basement prison accessible only by trapdoor. In 1315, the castle passed to the Butlers of Ormond, who held it for four centuries. The 4th Earl rebuilt the gate building in the 15th century, and in the 17th century a second-floor living area was added with pointed groined vaults, lancet windows, a garderobe, and a fireplace decorated with a hooded dog-tooth capital. Two three-quarter round towers complement the gate: the southwestern Ormond Tower, with its 17th-century plasterwork coat of arms, and the taller southeastern tower -- King John's namesake -- with three storeys.

In 1646, Owen Roe O'Neill stormed the castle and town during the upheavals of the Cromwellian era. By 1650, it had fallen to Cromwell himself and was briefly used by his son-in-law Henry Ireton. The castle changed hands again in 1703 when the Duke of Ormonde sold it to the Royal Hospital Kilmainham, and in 1722 local merchant John Damer purchased it. The castle was later used as a military barracks from 1798, housing 350 soldiers, and subsequently served as a school, a library, and a tuberculosis sanatorium. The roof collapsed in the 19th century, requiring extensive repairs in the 1850s. It was declared a national monument in 1892, registered as number 211.

In 1728, John Damer built a large Georgian house on the castle grounds in Queen Anne style. The three-storey-over-basement house features nine bay windows and one of only two Queen Anne style staircases surviving in Ireland. When demolition was proposed in the 1960s, Desmond Guinness -- heir to the brewing fortune and champion of Irish architectural heritage -- intervened. The Irish Georgian Society took a lease in 1973 and began restoration, which was continued by the Roscrea Heritage Society and completed in the 1990s with over 1.3 million pounds in funding from the national heritage service Duchas and Bord Failte. Today the house is owned by Tipperary County Council and has received both a European Museum of the Year Award special citation and an Irish Museums Trust Award. From the air, Roscrea Castle reads as a compact walled enclosure in the town center, the Georgian house nestled improbably within medieval walls.