Grallagh comes from the Irish Greallach, meaning a miry place, and the description fits. This small parish of around 400 acres in north County Dublin sits on marshy, low-lying ground, and the ruined medieval church at its centre occupies the highest point of the local graveyard, which is not saying very much. What makes the site quietly odd is its dedication: the church was built in honour of an Irish saint who spent his entire religious life on the Continent, died in what is now northern France in 978, and yet gave his name to a patch of wet ground outside the Pale centuries later.

St Maccallin, as the historian D'Alton described him in 1838, was an Irishman of the tenth century who became a Benedictine monk at Gorze Abbey in the Moselle region of France, rose to become Abbot of Waulsort on the Meuse in present-day Belgium, and eventually retired to the church of St Michael on the frontiers of Hainault, dying there on the 21st of January 978. The curvilinear bank and ditch enclosing Grallagh graveyard may indicate an early ecclesiastical enclosure, possibly belonging to a monastery founded here in the tenth century and connected to his cult. By the medieval period the chapelry had passed into the orbit of the Augustinian canons of Llanthony Priory near Gloucester, and the manor belonged to the Cruise family of The Naul. In 1393, Teigue O'Byrne granted lands in the adjacent townland of Mullahow to the rector in exchange for prayers for his soul. By 1615, the Royal Visitation of Dublin recorded the church as worth 20 marks, served by a curate named Terence Ivers. Fifteen years later, Archbishop Bulkeley's visitation noted that the churches and chancels were already ruinous and that fewer than eight people attended divine service in the parish. The 1654 Civil Survey recorded simply "the walles of a Chappell" on lands then belonging to Christopher Cruise, who lost everything in the 1641 rebellion.

When a survey was carried out in 1887, the west gable alone remained standing, ivy-covered and containing a small window; the outlines of the nave and chancel could still be traced. Today, the situation is much as it was then, only more so. The west gable, roughly 7 metres long and about 2.5 metres in height internally, still stands in roughly coursed masonry, though the window recorded in 1887 had already disappeared by the time photographs were taken in 1975. The rest of the church survives as a sunken rectangular area defined by grass-covered wall footings and occasional rubble. The east gable has been absorbed entirely into a concrete grave surround topped by a ringed high cross of early twentieth-century date, and several more recent memorials occupy what was once the interior. About 20 metres to the south, a holy well dedicated to the same saint sits in a carefully restored stone-roofed enclosure, worth a look once you have taken in the ruin.