Killasser, from the Irish "Cill Lasrach", meaning the church of Lasair, points to a female saint, Lasair, who is associated with this part of Connacht. The dedication alone places this site within a network of early medieval ecclesiastical foundations that once dotted the Irish countryside, many of them now reduced to field-edge rubble, half-remembered in placenames and the occasional carved stone.

Lasair is a figure who appears in Irish hagiography connected to the broader cult of saints in the west of Ireland, sometimes described as a daughter of the saint Ronan. Killasser parish takes its name from this church, suggesting the site held local religious significance well before the Norman period. Ruined churches of this type, often roofless shells of nave and chancel, sometimes retaining remnants of a Romanesque or early Gothic doorway or window, were frequently in continuous use from the early medieval period through to the post-Reformation era, when many fell into disuse as parishes were reorganised and new churches built elsewhere. The graveyard surrounding such sites often continued in use long after the church itself fell silent, which is how so many of these ruins have survived at all, tended at the margins by generations of families burying their dead.

The ruins at Knockmullin sit within this quiet tradition of layered use and gradual forgetting. Without detailed excavation records or architectural surveys available, the precise construction phases of the standing fabric remain unclear, but the site's age as a place of religious assembly is encoded in the very landscape around it, a parish named for a saint few outside Mayo would now recognise.