Augirein is located in the former province of Couserans some 30 km south-east of Saint-Gaudens and 25 km west by south-west of Saint-Girons. Access to the commune is by road D618 from Saint-Lary in the west which passes through the north of the commune and the village and continues east to Orgibet. Just east of the village is the hamlet of Terrefete. The commune is almost all rugged and heavily forested except for a small area in the north where the village is.
The Bouigane river flows through the north of the commune from west to east and continues to join the Lez at Audressein. The Ruisseau de Nede rises south of the commune and flows through the centre from south to north to join the Bouigane on the north-eastern border of the commune. Several tributaries rise in the commune and join the Ruisseau de Nede including the Ruisseau de Couledoux and the Ruisseau des Souls.
In 2010 the commune had 64 inhabitants. The evolution of the number of inhabitants is known from the population censuses conducted in the commune since 1793. From the 21st century, a census of communes with fewer than 10,000 inhabitants is held every five years, unlike larger towns that have a sample survey every year.[Note 1]
^At the beginning of the 21st century, the methods of identification have been modified by Law No. 2002-276 of 27 February 2002, the so-called "law of local democracy" and in particular Title V "census operations" allows, after a transitional period running from 2004 to 2008, the annual publication of the legal population of the different French administrative districts. For communes with a population greater than 10,000 inhabitants, a sample survey is conducted annually, the entire territory of these communes is taken into account at the end of the period of five years. The first "legal population" after 1999 under this new law came into force in 1 January 2009 and was based on the census of 2006.