It flows in the municipality of Sainte-Agathe-de-Lotbinière, in the Lotbinière Regional County Municipality, in the administrative region of Chaudière-Appalaches, in Quebec, in Canada.

The main neighboring watersheds of the River Armagh are: The Armagh River has its source in the municipality of Sainte-Agathe-de-Lotbinière at 4.6 kilometres (2.9 mi) south of the village.

This headland is located 0.8 kilometres (0.50 mi) north of the Palmer River, east of the source of the Rivière aux Chevreuils and 2.2 kilometres (1.4 mi) northwest of the municipal boundary of Saint-Jacques-de-Leeds.

From its source, the Armagh River flows on 7.7 kilometres (4.8 mi) divided into the following segments: The Armagh River empties on the east bank of the Filkars River at 4.8 kilometres (3.0 mi) east of the village center of Sainte-Agathe-de-Lotbinière.

The toponym Rivière Armagh was formalized on October 16, 1983 at the Commission de toponymie du Québec.