Fort, stronghold. Written Dun, Doon, Down — the same Irish word, dún, in the spellings English gave it.
Dún is a fort, and a bigger word than lios or ráth: a stronghold, a royal seat, a fortified headland. Dún Aonghasa on Aran, Dún Laoghaire, Dundalk (Dún Dealgan, the fort of Dealga), Dungannon, Donegal itself — Dún na nGall, the fort of the foreigners, a Viking base. Where a Dún- name stands there was somebody worth defending.
Source: P. W. Joyce, The Origin and History of Irish Names of Places (1869–1913) · Logainm ↗
Fort, stronghold: it is the Irish dún. Dún is a fort, and a bigger word than lios or ráth: a stronghold, a royal seat, a fortified headland. Dún Aonghasa on Aran, Dún Laoghaire, Dundalk (Dún Dealgan, the fort of Dealga), Dungannon, Donegal itself — Dún na nGall, the fort of the foreigners, a Viking base.
dún. English spellings of the same word include Dun, Doon, Down, because the names were written down by ear.
Dundalk (Dún Dealgan), Dungarvan (Dún Garbháin), Downpatrick (Dún Pádraig), Portadown (Port an Dúnáin), Dún Laoghaire (Dún Laoghaire), Dungannon (Dún Geanainn).