The name Killeavy is derived from Cell Sleibhe Cuilinn, 'the church of Slieve Gullion'.

The churches, situated on the east-facing lower slopes of the mountain are the reputed site of an important Early Christian Convent founded by St Moninna (also known as Darerca or Bline). In 450 she erected a wooden church here, which is said to have been replaced in 518 by a stone building. Although there are no remains of the early wooden church buildings, there are two stone churches at the site; the ruins of a thirteenth century building and of another many centuries earlier. Although plundered by Vikings in 923 A.D. monastic life continued and the site was occupied by Augustinian Nuns until 1542.

The large tree-lined graveyard is still in use today. There was a round tower here and there is a holy well to which there are still very large pilgrimages.

St Moninna was born in 432. Her father was Machta, king of the territory stretching from Louth to Armagh. Her mother was Comwi, daughter of one of the northern kings. According to tradition, Moninna, when a child, was baptised and confirmed by St. Patrick. It is said that Patrick came to her parent's house, blessed the family and predicted that Moninna's name would be remembered throughout time. She founded a number of convents in Scotland and England and also monastery of nuns in Faughart, Co. Louth. She later moved to a place near Begerin in Co. Wexford to be under St Ibar, before coming to Killeavy where she died in the year 518.

On the northern side of the cemetery at Killeavy Old Church there is a very large granite stone measuring seven feet long, five feet wide and about one and a half feet thick. This stone covers the supposed grave of St Moninna.