A long, rectangular church separated into nave and chancel by a broken cross-wall, extant only at ground level on the S side. This marks the foundations of the E wall of an earlier church.

The monastery was probably founded in the 6thc. by St Lurach. The Annals mention the deaths of some of its abbots and its ruination in the 9thc. It was also burned in 1135. It is listed by Cardinal Papero after the synod of Kells (1152) as a diocesan centre and suffragan of Armagh. In 1245 its bishop, Germanus, was chosen to be the Irish candidate for the vacant see of Armagh. The church was repaired a number of times following its damage during warfare in 1688. It fell into disuse and was eventually abandoned in 1819 when it was dismantled. Some of the stones were used to construct the new church. A major conservation programme was completed in 1984.

In structural terms the W doorway at Maghera is closely paralleled by the W doors at Banagher (Derry) and Aghowle (Wicklow), although neither have the elaborate sculpture found at Maghera.

The Crucifixion scene is the most complex to survive in Ireland from the 12thc. The inclusion of elements such as the Scourging of Christ and the apparent inclusion of the Two Thieves have no parallels in Irish art. However, the figure style, particularly that of the representations of Stephaton and Longinus is deeply rooted in Irish tradition, finding close parallels in the Tynan group of crucifixion plaques (Hamlin and Haworth, 1982). The closest parallels for interlace and geometric ornament are found on the Soisceal Molaise, a book shrine, originally from Devenish in Co. Fermanagh.