The element rath refers to a ringfort, the most common monument type surviving in the Irish landscape, and the presence of that word in the place name itself suggests the earthwork here was significant enough, and visible enough, to define the locality for centuries. There are estimated to be around 45,000 ringforts recorded across Ireland, yet each one represents a farmstead or enclosed settlement, typically dating from the early medieval period, roughly 500 to 1200 AD, where a family or small community lived within a circular bank and ditch for both practical protection and social display.

Ringforts of this kind were constructed by digging a ditch and piling the spoil inward to form a raised earthen bank, sometimes reinforced with a timber palisade or stone facing. The interior would have contained a dwelling house, outbuildings, and perhaps storage pits or a souterrain, an underground stone-lined passage used for keeping food cool or providing refuge. The rath at Rathcash sits within a Mayo landscape that was well settled during the early medieval period, and the survival of the place name into the present day is itself a form of archaeological record, a layer of evidence that outlasts the earthwork it describes.