[2] The medieval importance of Tuam was chiefly the result of its becoming the seat of the O'Connor High Kings of Ireland in the early 11th century.

[4] The first cathedral lasted only a few years and was destroyed by fire in 1184, recorded thus in the Annals of Lough Cé: "The great church of Tuam-da-Ghualann fell in one day, both roof and stone".

[5] After this, no rebuilding was attempted for some one hundred years, except that in the 13th century a small parish church was built on the site of an old monastery.

[4] This long delay was largely the result of the Norman invasion of Ireland, which left the city of Tuam in ruins.

[6] In the 14th century, a second St Mary's Cathedral was begun, a little to the east of the original building and incorporating into its entrance the remains of the 12th-century chancel and sanctuary.

[7] With the coming of the railway to Tuam in 1861, and the enlargement of the garrison, the town's Anglican population increased and this led to the building of a third cathedral on the site, designed by the architect Sir Thomas Newenham Deane, which was begun in about 1861 and completed in 1878.

The Bishop's Throne, the Pulpit, the Font and the Chapter Stalls were made of Caen stone and Irish marble.

[8] The Synod Hall stalls are reputed to have been in a Piedmontese monastery, and brought to Ireland by Edward Joshua Cooper, M.P., of Markree Castle, County Sligo, and were later presented to St. Mary's Cathedral.

[5] It also contains the High Cross of Tuam, a national monument which was moved to its present site in 1992, and a significant part of the 14th-century cathedral.

The Nave of O’Connor's cathedral collapsed in 1184 due to a fire, with only the stone chancel arch escaping.

The capitals are richly sculptured with a variety of interlaced traceries, something similar to those on the base of the High Cross in the Square.

The chalice bears the inscription, Ex dono Revernd mi Patris in Christo Sam Providentia Divina Tuamensis Archieppi & Feneborensis Epis Conaecieq Metropolitani.

Both the second chalice and the paten are hallmarked and bear the inscription, Ecclesiae Cathedralis Stae Mariae Tuamensis 1678.

These two items date from the Archbishopric of John Vesey (c. 1678 – 1716), There are also two flagons with a similar inscription, Ex dono Viri Venerabilis Honorabilis Thoma Vesey, Episcopi Laonensis Equitis Aurati quondam Archdiaconi Tuamensis in Usum Ecclesiac Cathedralis do Tuam & Gloriam SS & Individuae Trinitatis.