The Cross of Cong was made in 1123 to encase a fragment of the True Cross that was brought to Ireland and displayed in different places around the country. The cross is so-called because it was kept in the Augustinian Friary at Cong, county Mayo, for centuries.

The Cross of Cong represents the conclusion of a long tradition of distinctively Irish church metalwork. It is decorated in an impressive range of styles and can be considered to be one of the greatest treasures of the era.

It was made to enshrine a relic of the True Cross, known from written sources to have been acquired in AD 1122 by Toirrdelbach Ua Conchobair (Turlough O’Conor), High King of Ireland. The cross was designed for processional use, although it may have been mostly used as an altar cross. It has an oak core which is covered by plates of cast bronze. A large polished rock crystal on the front of the cross at the junction of the arms and shaft was intended to protect the relic, which does not survive. The rock crystal is set in a conical mount surrounded by a flange decorated with gold filigree, niello and blue and white glass bosses. The bronze plates on the surfaces of the cross are cast openwork and are decorated with ribbon-shaped intertwined animals in the Scandinavian-derived Urnes style.

The inscription on the sides of the cross identifies Toirrdelbach Ua Conchobair as the patron and Máel Ísú mac Bratáin Uí Echach as the craftsman. Two prominent churchmen are also mentioned. The inscription, which is in Irish, is framed by two identical lines in Latin that translate as ‘by this cross is covered the cross on which the creator of the world suffered’.