St. John’s Cathedral is almost certainly built on the site of a medieval hospital and parish church, founded in the 13th century in honour of the Trinity.
Some of this 13th century work is likely to be incorporated in the present west tower.
The Present Building
When architect Richard Cassels came to Sligo in 1730 to design Hazelwood House for the Wynne family, he was also commissioned to build St. John’s Church.
In designing the church, Cassels was greatly influenced by the basilica pattern in early Roman architecture.
In subsequent building modifications in 1812 and 1883, the external appearance was substantially altered through the replacement of the original Romanesque windows (with their round arches), the addition of battlements and small towers, and an extension of the chancel.
Some of the former Romanesque windows may still be seen in the west tower.
The Architect, Richard Cassels (1690-1751)
Cassels, who was born in Germany, was considered to be one of the greatest architects working in Ireland in the 18th century. In his adopted country his surname became anglicised to “Castle”.
He was responsible for the design of many of Ireland’s premier buildings of that era, including Leinster House (the present Irish Parliament building), Powerscourt House, Russborough House, Carton House, Westport House and the Rotunda Hospital.
He is known to have designed only three churches…Knockbreda Parish Church (Belfast), St. John’s (Sligo) and the now-demolished old Parish Church of Castlebar.
The Cathedral Church
When the Cathedral Church for the dioceses of Elphin and Ardagh, located in the village of Elphin, was abandoned in 1961 following severe storm damage four years previously, the Seat of the Bishop of Elphin and Ardagh was transferred to St. John’s Church, Sligo.
Accordingly, on 25th October 1961, the Church was constituted as the Cathedral Church for the dioceses of Elphin and Ardagh, under the name of the Cathedral of St. Mary the Virgin and St. John the Baptist.