The name, Inis-maige-samh, means the island of the sorrel plain. The founding saint, Ninnid, lived in the 6th century and according to much later sources studied with Finnian of Clonard and was a contemporary of Ciaran of Clonmacnois, Molaise of Devenish and Aiden of Ferns, but these sourc­es are too far from his lifetime to record more than traditions. One reminder of the saint’s local importance are place-names, like Knockninny, the prominent hill beside Upper Lough Erne, and Tober Ninny, the holy well overlooking the Lower Lough. An abbot of Inishmacsaint, Fiannamail, was killed in 718, and the monastery is unlikely to have escaped harm in the Viking raids of the 9th and 10th centuries. By the time of the 1306 taxation list Inishmacsaint was a parish church, occupying the early monastic site. Rectors of the church appear in 15th- and 16th-century annals: the O’Flanagan family was prominent, and in 1426 Nicholas O’Flanagan held the rectory jointly with Devenish. A roofless church on the island is shown in a 1609-10 map, and it was probably still ruined in 1622, when there was a complaint that ‘the old church standeth in an inconvenient place’. The Protestant population seems to have worshipped in Sir John Dunbar’s church at Der­rygonnelly until a new church was built at Church Hill in 1688, to be replaced in turn in 1831 by the present Inishmacsaint parish church at Benmore.

The ancient church shows work of three periods. Earliest is the west part, which may represent a small, pre-Romanesque church, perhaps of the 10th or 11th century. In a major remodelling the W. door was blocked and a new S. door opened, and the church was extended eastwards to its present length. This may have happened in the early 13th century, by which time it was a parish church. There are two cupboards near the E. end, where the altar once stood. In the later middle ages the S. window was altered. No other windows survive, but loose architectural fragments remain from the missing features.
The cross stands just SW. of the church. It is tall, thin and rather plank-like, its two stones joined at the head with a mortice-and-tenon joint, visible in the break on the E. side. The head is of unusual form, lacking a ring and with the arms hollowed in very shallow curves. A third stone is missing to complete the top member: a mortice hole exists to hold it. In good light it is possible to see faint traces of rectangular panels on the ends of the arms and under the arms, but the general impression is severely plain. The cross is difficult to parallel and to date: between the 10th and 12th centuries seems likely, but it could be later.
St. Ninnid’s bell, traditionally made by Senach the smith of Derrybrusk, was kept in the parish in the 17th century. By the 19th it was at Castle Caldwell, and it is now in the National Museum, Edinburgh.