The church has coeval nave (10.15 m x 5.15 m internally) and chancel (3.55 m x 2.50 m internally), the chancel at a slight angle to the nave. A later sacristy is incorporated to the N of the nave. A doorway is centrally situated in the W wall of the nave. The nave is lit by four windows, one above the doorway in the W gable and three in the S wall. Only the easternmost of the S windows retains any original features, the others were renovated, possibly during the 18thc.

Originally known as Inis Loch Cré, the name Mona Incha was only given to the site in the in later centuries. The earliest monastery here was founded by St Cainneach of Aghaboe in the 7thc as a hermitic settlement. Column Mac Fergus died on Inis CrÈ in 788 as did Elarius, anchorite and scribe, in 807. The latter was St Elair, or Hilary who was patron of the monastery in which the Culdees were instituted in around the 10thc. In 902 Flaithbertach, King of Cashel, came here on pilgrimage. An Augustinian community was established at Mona Incha during the mid-12thc.

After his visit to Mona Incha in 1185–86, Giraldus Cambrensis describes it as follows: 'There were two islands, the larger with a church (St Hilaire) the smaller with a chapel (St Colum) served by a few coelibs who were called Coelicae or Colidei (Culdee), and no female of man or beast could live on the island'.

In 1397 the monastery, containing a prior and eight canons, was taken under the Pope's protection and released from paying Episcopal dues because of its extreme poverty. At about the same time the Book of Ballymote described Mona Incha as the 31st wonder of the world! In c.1485 the prior and convent finally moved to Holy Cross at Corbally, as the canons found the vapours from the marshes surrounding the island unhealthy. Following the Dissolution the church was used as a penal chapel. The site is currently under the care of Duchas, the Heritage Service.

Some of the carving, particularly that of the door jambs is similar to that at the Nun's church at Clonmacnoise (Offaly). The zoomorphic hindquarters represented on the N springer of the second order of the chancel arch find a parallel on one of the springers of the E window at Annaghdown cathedral (Galway). The squared patrae on the outer order of the doorway find an almost identical parallel at Clonfert (Galway), although at Clonfert the floral decoration of the motif does not vary. The closest comparisons to the chevron ornament are found at Killaloe and Tuamgraney (Clare). These comparisons suggest a construction date of c.1180, perhaps to coincide with the establishment of the Augustinian canons at the site.

McNab has compared the cross to that at nearby Roscrea. She interprets the unpierced upper part of the cross as an indication that it is unfinished, and speculates that it may have been intended to be a copy of the Roscrea monument that was never completed. The base of the cross is comparable to bases belonging to the Ossory group of crosses, usually dated to between the 8thc and 10thc. The cross is shown as standing and complete in a 1790 etching in Ledwich's Antiquities of Ireland, although the ring is not pierced both above and below the arms and so its accuracy is dubious.