[2] It was named after the city and county of Armagh in Northern Ireland.

At the 2000 census,[4] there were 3,988 people, 1,532 households and 1,118 families residing in the township.

The population density was 43.0 inhabitants per square mile (16.6/km2).

There were 1,956 housing units at an average density of 21.1/sq mi (8.1/km2).

22.8% of all households were made up of individuals, and 11.6% had someone living alone who was 65 years of age or older.