t is the only land-locked municipality of the province. According to the 2000 census, it has a population of 45,507 people in 8,639 households.

Earlier than 1847, Irosin was a far inland valley unspoiled by human settlers and covered by vast forest expanse teeming with wild animals. Irosin was among the last places in the Philippines to be populated. Most of the natives during the early 1800s lived along the coastal areas where it was more accessible to commercial, travel and communication activities at the time. The place have been an undisturbed sanctuary for wildlife until settlers from Bulusan in search of wild quarry and rattan came. 

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