This is a rath, the most common type of monument surviving in Ireland, yet one that still manages to feel anomalous when you come across it unexpectedly. Raths were essentially farmsteads, built during the early medieval period, roughly between the fifth and twelfth centuries. A farmer of moderate means would have enclosed his home, his family, and his livestock within a circular bank and ditch, less for military defence than to signal status and to keep animals from wandering. Tens of thousands once dotted the Irish countryside; a few thousand survive in recognisable form today.

The specific history of this particular enclosure in Castlereagh remains largely undocumented in any publicly available form. What can be said with confidence is that the townland name itself carries historical weight. Castlereagh, from the Irish Caisleán Ríabhach, meaning the grey or brindled castle, suggests a layered past in which different periods of settlement left their marks in the same stretch of ground. The rath would predate any castle implied by that name by several centuries at least, representing an older pattern of rural life that the Normans and later settlers built over or around rather than erased entirely. Mayo as a county contains a substantial number of such earthworks, many of them still unexcavated and holding their contents, broken pottery, animal bone, traces of timber structures, beneath the soil.

Because so little detailed information is currently available for this particular site, a visitor would be wise to approach with modest expectations and a general familiarity with how raths present in the field. The characteristic features to look for are a circular raised bank, sometimes doubled, and a slight depression where a ditch once ran. The interior is often level and noticeably clear. In areas of improved farmland, raths can be surprisingly well preserved simply because generations of farmers have avoided ploughing through them, partly out of practical inconvenience and partly out of a cultural reluctance, still alive in rural Ireland, to disturb what the old people called fairy forts.