According to extant records St. Patrick, accompanied by Bronus, crossed from Tireragh by way of Traigh Eothaile, (alias The Strand at Streamstown) to Irai (alias Cuil Irra), in which place was the Cottage of Bronus. At the extreme north-western point of the peninsula Patrick marked out Caiseal Irra, inside which St. Bron, or Bronus, built his primitive church, Cill-Easpaig-Bron. St Bron a beloved disciple of St. Patrick, is said to have been a native of Coolera. The annalists tell us that he died in the year 511 and is venerated on the 8th of June in Caiseal-Irra, in the country of Tir-Fiachra.
The existing church remains at Killaspugbrone, in plan and style of masonry, present indications of considerable antiquity - for example, the location of the original doorway in an elevated position in the centre of thewestern gable. In time this was replaced by an entrance in the south side-wall, a derivation from custom rendered necessary by the situation of the church on the seashore and its consequent exposure to the prevailing westerly winds. At the time of the insertion of this doorway further alterations would seem to have been made in the fabric of the church; amongst others was a pointed recess fashioned in the thickness of the northern wall, at a little distance from the eastern gable and nearly facing the ancient altar stone which still remains in place. The eastern window is extremely elegant in style with its mouldings and inclined sides. While the masonry surmounting the slopes of the western gable was probably built as a screen for the roof of the church to shield it from the prevailing winds blowing in from the Atlantic.
It is more likely that the present ruin dates from the 11th or 12th centuries rather than the 5th. Part of the fabric is thought to date from as late as the 15th century. It is not in the least degree probable that any portion of the structure erected by St. Patrick remains, at least above the ground. wrote W. F. Wakeman. However, the existing remains in place and style of masonry present indications of considerable antiquity. . . In the course of centuries it was doubtless rebuilt more than once.
Killaspugbrone was mentioned in the 1306 Taxation Roll and was of sufficient importance to have the death of its Vicar. Peter O'Tuathalain recorded by the Four Masters in that very year. References can also be traced in Papal Records under the years 1404; 1405; 1408 and 1427. As a parochial unit Killaspugbrone was never outstanding, despite the record of its Vicar's death in the Annals. Some importance it undoubtedly had but this was probably derived from the qualities of the Vicar rather than the status of the vicarage itself. The value of the Vicarage in 1405 was less than 4 marks - one of the poorest in the whole diocese of Elphin. It was valued at 4 marks in 1427. sufficient only to give a bare subsistence to the Priest in charge of the Parish.
An Inquisition taken on August 18th, 1585, sought to list the possessions which belonged to Killaspugbrone when the newly established Protestant church seized them and dispossessed the original owners. This valuation revealed that the Vicarage of Killaspugbrone was valued higher than others in the Sligo area but lower than that of Drumcliffe. In a Deed dated August 9th, 1605, the then Bishop of Elphin, John Lynch, demised to Henry Lynch of Galway a long list of See Possessions. Amongst these were the mensal lands (one quarter) in the Parish of Killaspugbrone, which were leased at a yearly rent of Ten Shillings for 99 years.
The Church fell into disuse circa 1680. In 1811 the ruins were repaired by the Vestry of St. John's who also built the wall around the graveyard in 1814. The chancel of the present St. Anne's church incorporates a few stones from the old structure in addition to the ancient baptismal font of rough stones.