These coastal forts, sometimes called cliff castles, belong to a building tradition stretching back through the Iron Age and beyond, in which a narrow neck of land was cut off from the mainland by one or more earthen banks and ditches, leaving the cliffs themselves to do the defensive work on the remaining sides. The result was an enclosure that required minimal effort to defend and offered an almost unassailable position for whoever held it.

Cill Ghallagáin, whose name suggests an early ecclesiastical connection, sits in a part of Mayo where prehistoric and early medieval remains are not uncommon, scattered across a coastline that has been inhabited for thousands of years. Promontory forts of this kind are found all along Ireland's western seaboard, and their precise dates and functions are often difficult to pin down without excavation. Some were permanently settled; others may have served as temporary refuges during periods of raid or conflict. Whether the earthworks at Cill Ghallagáin reflect a single phase of construction or were added to over generations remains unknown.