Irosin, officially known as the "Municipality of Irosin", is a municipality in the province of Sorsogon, Philippines. Dating back to 1847, Irosin was a…Irosin, officially known as the "Municipality of Irosin", is a municipality in the province of Sorsogon, Philippines. Dating back to 1847, Irosin was an inland valley devoid of human inhabitants and covered by a vast forest with thriving biodiversity, making it one of the last places in the Philippines to be occupied by settlers. In the early 1800s, most natives lived along the coastal areas, allowing them greater access to commercial, travel, and communication opportunities.