The Municipality of Irosin lies at the foot of Bulusan Volcano, the tail end of the Sierra Madre Mountain Ranges at the southern tip of Luzon Island. It is located at the center of Sorsogon Province bounded by coordinates 12.7780° N, 123.9521° E and 12.6373° N, 124.0962° E. It is nestled in a caldera with a valley floor that is surrounded by mountains and peaks making the town the only landlocked inland municipality of the province.
Irosin is approximately 643 kilometers south of Manila, 106 kilometers from Legaspi City and 43 kilometers from Sorsogon City. The adjoining towns of Bulan and Matnog, the gateway to Masbate, Visayas and Mindanao are 20 and 23 kilometers away respectively.
Irosin’s official gross land area is 14,987 or 149.87 square kilometers. The recent CLUP land use study suggests that the land area is 15,110.67256 hectares.
The municipal boundary of Irosin has a perimeter distance of 52.52 kilometers. On the northwest is the municipality of Bulusan with a 10.47 kilometers shared boundary, on the southwest is Sta. Magdalena with 8.6 kilometers boundary, on the south is Matnog with 9.47 kilometers boundary, on the southeast is Bulan with 9.71 kilometers boundary and on the northeast is Juban with 14.27 kilometers boundary.
Irosin is comprised of 28 barangays of which five are classified as urban barangays while 23 are rural mostly farming barangays. The poblacion or urban barangays of Bacolod, San Agustin, San Juan, San Julian and San Pedro are located in the valley floor at the center of the municipality. Most of the rural barangays are dispersed along the periphery of the valley while Salvacion, Casini, Cawayan and Liang are in hills and mountains.
To the north of urban areas are the barangays of Bagsangan, Monbon and Cogon. At the northeast are Sto. Domingo, Patag and Mapaso. To the east is Cawayan and to the southeast is Tabon-tabon. The barangays of Carriedo and Liang are in the south and at the southwest are Buenavista, San Isidro, Salvacion and Casini. To the east of the poblacion are Macawayan, Batang, Gumapia, Tongdol, Gabao and Bulawan and to the northwest are Tinampo, Gulang-gulang and Bolos.
The urban barangays has a land area of 318.333 hectares or only 2.11% of the total land area while the rural barangays has an aggregate area of 14,792.342 hectares or 97.89% of the total land area. The largest barangay in terms of area is Tabon-tabon with 1,837.4394 hectares or 12.17% of Irosin’s total land area followed by Cogon and Cawayan with areas of 1,637.4932 and 1,163.3442 hectares respectively. The smallest barangays are in the urban barangays with San Juan having only 10.5334 hectares of land, followed by San Agustin with 15.1620 hectares and San Julian with 17.3498 hectares of land.