The element "rath" refers to a ringfort, the circular earthwork enclosure that was the standard form of rural settlement across Ireland during the early medieval period, roughly from the fifth to the twelfth centuries. Tens of thousands of these structures were built, yet each one represents a farmstead where a family lived, kept livestock, and organised their world within a raised bank and ditch. The name Rathgranagher has preserved the memory of one such enclosure even as the landscape around it has changed beyond recognition.

Ringforts, or raths, were typically formed by one or more concentric earthen banks, sometimes reinforced with stone, enclosing a circular area where a house or houses once stood. The enclosure served less as a military fortification and more as a boundary marker and a deterrent to cattle raiders, which was a persistent concern in early Irish pastoral society. The place name itself is the most direct evidence we have that a rath once defined this particular patch of Mayo ground, and place names of this kind have long outlasted the physical remains they describe, sometimes by many centuries.