It is a quiet mislabelling, and the real story sits just to the northwest of the graveyard where an earthen bank curves around a roughly circular area about 65 metres across. That bank, easy to miss against the surrounding fieldscape, is thought to represent the remains of an ecclesiastical enclosure, the kind of bounded precinct that would have defined an early Christian monastery. The enclosure boundary is so old that part of it has been absorbed into the townland boundary itself, the curving line on the north side now separating Glassavullaun from Glassamucky to the north and Ballymorefinn to the west.

The monastery it once enclosed was Kilmasantan, whose Irish name, Ceall Easpaig Shantáin, means the church of Bishop Santán. The saint after whom it is named was probably active in the sixth century, and his family connections were unusually far-reaching. According to the genealogies studied by Ó Riain and others, Santán was the son of Samuel Ceanníseal, a figure whose name translates as 'head-bent' and who appears in Welsh sources as Sawyl Benisel. His brother was the pilgrim Madóg, a name borrowed from the Welsh Madawg, and his mother was Deichtir, daughter of Muireadhach Muindearg, king of Ulster. The saint's reach extended beyond Ireland: a parish on the Isle of Man is named Kirk Santan after him, and there, as here, his memory was later quietly absorbed into the cult of St Ann, which is how that misleading chapel name came to be written on the maps. The monastery survived long enough to have an abbot recorded by name; the Annals of the Four Masters note the death in 962 AD of Caenchomraic, abbot of Cill Easpuig Sanctan.

The site sits on a west-facing slope above the east bank of the River Dodder, in the upland valley of Glenasmole in the Dublin Mountains. Inside the graveyard gate, immediately to the north, a medieval font still stands in place. The holy well associated with the site, originally St Santan's Well and now known as St Ann's Well, lies about 175 metres to the northeast. The earthen enclosure bank adjoins the northwest side of the graveyard and is best read by walking the field boundary slowly and watching how the curve of the ground differs from a straight agricultural line. The dual feast days of St Santán, the 9th of May and the 10th of June, would once have drawn local observance to this valley; the well and the enclosure together suggest how much of that early monastic landscape is still, faintly, legible on the ground.