Killeshin Union is made up of four churches: Castletown, Cloydagh, Killeshin and Mayo.
Mayo church is located 8 miles from Carlow, 6 miles from Castlecomer and 1.5 miles from the village of Crettyard. It is situated in the ancient parish of Kilabban in the Diocese of Leighlin. It is believed that an older church originally stood on the site of the present church, possibly the location of a pre-Reformation place of worship. The church of Mayo was built in 1812, designed to be a chapel of ease to Castletown church. The building costs were £738, borne by the Board of First Fruits[i]. It seats 130 persons and its wall memorial plaques bear witness to the Carter, Edge and Stapleton families. The parish then became known as Kilabban Union. (ie Castletown and Mayo) In 1813 the population of Kilabban Parish was recorded as being 8996 Roman Catholics of 1525 families and 474 Protestants of 68 families. The older graves in the graveyard included the last resting place of the Hovendons, the Goulds and Lord Galmoy of Garrendenny Castle.
According to the Ecclesiastical Report 1837 there was one rector and two curates in the parish . Church services were held in each church twice on Sundays in summer, once a Sunday in winter and on every principal festival. The Sacrament was administered monthly and on the festivals.
The use of curates ceased in 1915 with the rector living near Ballickmoyler. In the vestry minutes of 12th November 1959, it is stated that the Commission for Sparsely Populated Areas had finally decided that Killabban should be joined with the Luggacurran district of Timahoe Parish. Luggacurran was amalgamated with Mayo and Castletown. Reverend A. E. Crawford was instituted in Luggacurran on November 23rd 1961. Fourteen years later, the Diocesan Council decided that there should be yet another arrangement of parishes. Luggacurran became part of Stradbally Union.
The amalgamation of Killabban Union with Killeshin and Cloydagh parishes took place in 1978 with the Rev Dermot McKenna as rector. This new group became known as the Killeshin Group of Parishes.
In 2011 the group became Killeshin Union, there is now one vestry for the whole union.