Ruined church of simple rectangular plan, with an internal length of approximately 27.5m. The 12thc building was considerably shorter, since there is evidence to show that the chancel was an addition. According to Comerford, the E end of the building was adapted as the local Protestant church at the beginning of the 18thc., alterations which may have involved the demolition of the Romanesque chancel arch, if such a feature existed. Large sections of the N and S walls are now missing. The E wall, detached from the rest of the ruins, contains a small late Gothic window, comprising a pair of ogee-headed lights. The W wall, which has antae at the angles, contains one of the most delicately worked Romanesque portals in Ireland.

There is historical evidence for a round tower, which was deliberately demolished in 1703. According to Comerford (1882), the 'ornamental stones of the doors and windows' of the tower could still be seen in 'little houses in the neighbourhood'. None of these carvings have been located. Given the ornate treatment of the W doorway, it is likely that the church once had a similarly decorated chancel arch.

Killeshin was the site of an important early Irish monastery, founded in the 6thc. The annals record the names of a number of abbots and distinguished scholars associated with the monastery in the ninth, 10thc. and 11thc. There are no records for the 12thc., except under the year 1147, when Dowling's annals (Butler, Annals of Ireland) record the achievements of Cogganus of Killeshin, who wrote accounts of St Malachy of Armagh and St Bernard of Clairvaux. The size of Killeshin and its adjoining settlements can be judged by an attack on the monastery in 1042, when the oratory was broken, one hundred men were killed and four hundred taken prisoner (Annals of Ulster). Monastic life at Killeshin probably came to an end shortly after the Anglo-Norman conquest of Leinster in 1169-70. The church evidently remained in use during the later middle ages, serving the needs of the local parish.

Lord Walter Fitzgerald's article on Killeshin (1909) includes a photograph of Killeshin in 1880, showing the W gable covered in ivy.
There is also an undecorated, roughly cut, circular font.

The inscriptions on the portal at Killeshin have been the subject of variant readings and much debate. The main inscription appears to have started on the N side, on the S face of the outer order, and to have run continuously around the portal until the W face of the third order. Along the N abaci, Macalister (1945) believed the letters should read OR DA ...D.AR......I LAGEN, which he interpreted as OROIT DO DIARMAIT RI LAGEN, pray for Dermot King of Leinster. If this is accurate, it presumably refers to Dermot MacMurrough, King of Leinster between 1126 and 1166. The reading of the letters LAGEN is not in doubt, and if this is a reference to to a king, the style of the doorway indicates that this can only be Dermot MacMurrough, providing a terminus ante quem for the portal of 1166. This identification was not made by earlier writers, since at the time the Killeshin portal was felt to pre-date Dermot's reign. On the southern abacus, the letters are even more ambiguous and, as Macalister remarked, the restoration of the inscription as a whole seems hopeless.
On stylistic grounds, the portal of Killeshin can be dated somewhere between 1145 and 1165. The bulbous bases have close similarities with those at the Cistercian abbey of Baltinglass, founded by Dermot MacMurrough in 1148. There are also many parallels with the sculpture of St Saviour's church at Glendalough, supposedly founded by Laurence O'Toole when he was abbot between 1154 and 1162. The dynastic associations of the monastery, which have been studied by Edel Bhreathnach, help to support a date in the mid-12thc. Killeshin fell within the territories of the Ui Bairrche; in 1141 at the instigation of Dermot MacMurrough seventeen of the nobles of Leinster were killed or blinded and these included three sons of Mac Gormain of Ui Bairrche. It is likely that in the ensuing years Dermot assumed direct control over the Ui Bairrche and their lands. The reconstruction of the church, with its flamboyant doorway, incorporating international Romanesque features, may indirectly be a consequence of these events.
The sculpture of the portal is remarkably eclectic in its choice of motifs, which include features from early Irish art, Scandinavia and contemporary European Romanesque. Some of the panels of zoomorphic interlace are particularly fine examples in the Irish-Urnes style. The human faces, especially those in profile, recall the work of Irish illuminators, and Maire MacDermott has drawn attention to the similarities between the tiny animals on the archivolts of the third and fourth orders and those on the Kells crosier (British Museum). The beast heads spewing forth foliage represent a motif that is widely dispersed in Europe; parallels can be found at Verona cathedral, the Prior's Doorway at Ely, and the wooden doorway from Ulvik, now at Bergen.
In other respects the portal reflects common trends in Hiberno-Romanesque. It is one of seven portals with a tangent gable and the use of human heads on the angle of a capital is a frequently used device, particularly in Leinster (Kilteel, Timahoe, Duleek, Glendalough [St Saviour's, Priests' House], Rahan, and, outside Leinster, Inisfallen, Inchagoill and Annaghdown). Two types of human head were employed on the portal. One type with beard and moustache, typified by the keystone on the outer order, has similarities with an unprovenanced head in the Sainsbury collection in Norwich. There is little doubt that the Killeshin sculptor (or sculptors) were also responsible for the portal of the round tower at Timahoe, and further study may confirm the presence of the same workshop elsewhere in Leinster. Carvings at Baltinglass and Glendalough are noticeably similar in style. The curious chevron pattern on the face of the outer order, in which the angles are replaced by circles, recalls a motif found occasionally on early Irish grave slabs: see for example slab number 2 at Glendalough.