The great fort at Dún Aonghasa on the island of Inis Mór.
The Irish word dún refers to a fort or a King’s residence. The word also appears in placenames spelled as don, doon or down. Historically the term was applied to a range of monument types ranging from ringforts, larger promontory forts and hillforts that were built over a period of two thousand years. Ringforts are roughly circular or oval areas with a diameter of 20-50m, surrounded by an earthen bank with an external fosse as at Dúnheeda Fort in Co. Meath. Sometimes these forts have two or three banks and fosses. Ringforts functioned as farmsteads and date broadly to the second half of the first millennium AD. In other cases dún can refer to much larger forts with massive stone walls such as Dún Dúcathair, Dún Eoghanachta and Dún Aonghasa on the island of Inis Mór. Dún Aonghasa is now known to have been constructed in the Late Bronze Age while Dún Eoghanachta was built in the early medieval period. The term was also applied to some of the very large hillforts such as Dún Ailinne, the royal site for the Kings of Leinster. At Dún Ailinne a large bank and internal ditch enclosing an entire hilltop was built in the Iron Age.